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© 2003 The Journal of Bone and Joint Surgery, Inc.
What's New in Hip Arthroplasty
Michael H. Huo, MD Total hip arthroplasty remains one of the most frequently performed reconstructive procedures in orthopaedic surgery. The greatest challenges facing us today are the escalating volume of revision surgery and the increasing number of complications as the duration of in situ service of the implants continues to increase. For the current update, we reviewed articles from three major journals: The Journal of Bone and Joint Surgery (American Volume), Clinical Orthopaedics and Related Research, and The Journal of Arthroplasty. We also reviewed the abstracts presented at major meetings, including the annual meeting of the American Association of Hip and Knee Surgeons (2002), the annual meeting of the American Academy of Orthopaedic Surgeons (2003), The Hip Society Fall Meeting (2002), and The Hip Society Open Meeting (2003). We also reviewed selected scientific papers and abstracts especially pertinent to certain topics from other sources. We have organized the information into eight sections: (1) fixation with cement, (2) fixation without cement, (3) revision surgery, (4) complications, (5) surgical techniques, (6) articulation and wear, (7) growth factors and biology, and (8) outcome analysis and practice management. Fixation with Cement Fixation of the femoral stem with cement remains popular in clinical practice. The durability of cement fixation has been remarkable even after long-term follow-up. Callaghan et al. reported the thirty-year results of Charnley total hip arthroplasties performed with use of early cementing techniques. The overall revision rate was 13% for the entire group and 25% for patients who were alive thirty years after surgery. The cup revision rate was 6.4% overall and 24% for patients who were alive thirty years after surgery. The stem revision rate was 1.2% overall and 8.8% for patients who were alive thirty years after surgery. These data should be used as a benchmark with which to evaluate the efficacy of other designs. Lewallen et al. reported the stem-survival rates associated with 1411 total hip arthroplasties that had been performed with use of the same cup design over fifteen years. Thirteen different stems were used, both with and without cement. The five, ten, and fifteen-year stem-survival rates were 97.1%, 90.4%, and 82.4%, respectively. Two cemented stem designs and one cementless stem design fared worse than the others. These data provide a foundation for the assessment of future implants and it is hoped that they can be used for the early identification of outlier implant designs with poor durability. Controversy remains with regard to whether a matte-finished stem with cement fixation is at a higher risk of failure, as was reported by the senior author last year. Lachiewicz and Messick 1 reported the results of seventy-five hybrid total hip arthroplasties that had been performed with a precoated stem and third-generation cementing techniques after a mean duration of follow-up of ten years. No stem was revised or loose. The prevalence of femoral osteolysis was 4%. Sanchez-Sotelo et al. 2 reported the results of 256 total hip arthroplasties that had been performed with use of a stem with a matte finish and second-generation cementing techniques after a mean duration of follow-up of 15.4 years. The fifteen-year survival rate was 92.2% with aseptic revision as the end point and 90.1% with mechanical failure (including radiographic signs of loosening) as the end point. Younger age (less than fifty years) was the most significant factor associated with revision and mechanical failure (p < 0.0001). In a prospective, randomized trial, Vail et al. reported no difference, after four to seven years of follow-up, between polished (Ra 4) and grit-blasted (Ra 110) stems of the same geometry that had been inserted with cement with use of third-generation techniques. The five-year survival rate was 99.5% for both stem types. Moreover, there was no difference between the groups with regard to the Harris hip score, the Western Ontario and McMaster University Osteoarthritis Index (WOMAC) score, or the Short Form-36 (SF-36) score. Gie presented strong evidence in support of the use of polished stems. The Exeter stem has been in use for more than thirty years. The overall rate of aseptic loosening was 2.8% after the initial 433 total hip arthroplasties performed between 1970 and 1975. The stem was changed to a matte-finish surface in 1976, and the revision rate increased to 10% in the next ten years. Gie also followed 325 total hip arthroplasties that had been performed with a modular polished stem in the 1980s. There had been no stem revisions after a mean duration of follow-up of fourteen years, and the femoral osteolysis rate was 0.5% among living patients. Another controversy is whether the use of a stem with an increased offset will result in a higher failure rate as a result of a potentially greater bending moment. Bourne, Rorabeck, and Guerin reviewed their experience with 675 total hip arthroplasties after one to five years of follow-up. Forty-five percent of the procedures had been performed with use of a high-offset stem. Fixation had been performed both with and without cement. There was no difference between the normal-offset and high-offset groups in terms of the Harris hip score, the WOMAC score, limp, the need for walking aids, or the rate of revision. Longer follow-up is necessary to determine if differences in failure rates will become apparent. The principal mechanism of stem loosening is cracks in the pores within the cement mantle and subsequent debonding. Porosity-reduction techniques introduced over a decade ago have become a standard part of surgical technique. Another method for improving fixation at the cement-stem interface is preheating the stem. Iesaka et al. 3 reported that preheating the stem from room temperature to 50°C resulted in shorter polymerization time (p < 0.01), greater shear strength at the cement-stem interface (p < 0.001), and reduced porosity (p < 0.001). The initial increase in shear strength was 53%. This increase was even greater (155%) after simulated aging of the cement mantle. It is unclear whether this technique will further improve the durability of cemented stems with long-term cyclic loading in vivo. Moreover, it is unclear whether any deleterious clinical effects will occur as a result of the higher cement-bone interface temperature when the stem is preheated. It has been postulated that stair-climbing may be especially detrimental to cemented stems because of the high torsional loads that are created. Stolk et al. 4 , using a finite-element-analysis model, demonstrated that stair-climbing was associated with a sixfold increase in damage to the femoral cement mantle. There was a 47% increase in cement damage even when stair-climbing made up only 10% of the activities. Moreover, the adverse changes were predicted to involve the entire cement mantle rather than just the proximal region and the region at the distal tip of the stem as in the case of level walking. Fixation without Cement Cementless stem fixation has undergone substantial evolution over the past two decades. More data have become available in support of its clinical efficacy and durability. Waddell et al. reported on the trends of implant selection in North America from 1995 to 2001 with use of data from the North American Hip and Knee Registry, in which 10,299 primary total hip arthroplasties had been enrolled at the time of the study. Cement was used for fixation of the stem in 66.2% of the procedures in 1995, compared with 38.6% of the procedures in 2001 (p < 0.0001). Similarly, the percentage of procedures in which cement was used for fixation of the cup decreased from 12% in 1995 to 7.8% in 2001 (p = 0.002). This trend was attributed to improvements in design and surgical technique, coupled with increasing amounts of data supporting the clinical success and durability of cementless fixation.
Stem It has become increasingly apparent that the clinical results associated with cementless stems are influenced by several important factors, including geometry and the extent of coating. Durable fixation and a low rate of thigh pain consistently have been reported in association with tapered designs. Teloken et al. 6 reported excellent fifteen-year results in a study of forty-nine total hip arthroplasties that had been performed with a tapered cobalt-chromium stem with a proximal porous coating. The mean Harris hip score was 92 points, and the prevalence of thigh pain was 2%. No stem was revised for loosening, while two were judged to be loose radiographically. Most importantly, no stem that was judged to be bone-ingrown at two years after surgery progressed to loosening. Garcia-Cimbrelo et al. 7 reported good eleven-year results in a study of 104 total hip arthroplasties that had been performed with a tapered niobium-titanium stem with a rectangular cross-sectional geometry. No stem was revised. However, the prevalence of stem subsidence was 11.5%. Reitman et al. reported good thirteen-year results in a study of ninety-two total hip arthroplasties that had been performed with a tapered titanium-alloy stem with a plasma-spray coating. No stem was revised. The prevalence of stem subsidence was 7.6%. The important finding was that stem subsidence was no more common in femora with enlarged geometry than in femora with a smaller canal diameter. Longer-term follow-up of these series should provide additional data with regard to bone remodeling around a tapered design. Aldinger, Sabo, and Breusch specifically analyzed the bone-remodeling around a tapered stem with use of dual-energy x-ray absorptiometry. The authors followed twenty-six patients prospectively for as long as seven years after surgery and also analyzed the bone-mineral density in another thirty-five patients at thirteen years after surgery. No major degree of stress-shielding was found in any patient. There was minimal change in overall bone-mineral density between one year and seven years. The changes at thirteen years paralleled those observed in the prospective study group at seven years. The greatest reduction in bone density occurred in the proximal regions (Gruen zones 1 and 7). These data reflect the preservation of femoral bone density around one particular tapered stem design after intermediate-term follow-up. At the recent Hip Society meeting, Engh Jr. et al. presented the results of 129 total hip arthroplasties that had been performed with use of an extensively coated cylindrical design and followed for a minimum of fifteen years. The overall rate of stem revision was 3%. The prevalence of femoral or pelvic osteolysis was 24%. Stem loosening was not associated with osteolysis, whereas cup loosening was associated with osteolytic lesions measuring >1.5 cm 2 . D'Antonio et al. reported on 227 total hip arthroplasties that had been performed with a hydroxyapatite-coated tapered stem and followed for a mean of 12.2 years. The mean age of the patients at the time of surgery was 51.8 years. Only one stem was revised for aseptic loosening, at 9.5 years. The rate of mechanical stem failure was 0.4%. The prevalence of severe thigh pain was 2%. All femoral osteolytic lesions in that series were limited to the proximal zones at the time of the most recent follow-up. Long-term data from these two series should serve as a benchmark for the evaluation of other stem designs in the future. Flexible femoral stems with lower stiffness were reintroduced into clinical use in the hope of reducing the stress-shielding effects of the implant. Poor fixation and high rates of revision had been commonly noted in association with older designs. Karrholm et al. 8 reported the preliminary results of a prospective, randomized trial in which patients received either a traditional porous-coated stem or a porous-coated composite stem with reduced stiffness. Radiostereometry demonstrated stable fixation of both types of stems, and there was no difference between the groups in terms of clinical outcome measures. Dual-energy x-ray absorptiometry revealed that the loss of bone mineral was significantly reduced around the composite stems at two years in Gruen zones 1, 2, 6, and 7 (p < 0.0005 to 0.04). Moreover, the patients who had received the composite stem had significantly fewer sclerotic lines surrounding the stem (p < 0.002) and at the tip of the stem (p = 0.001). These early results are encouraging with regard to the reduction of stress-shielding effects. The long-term clinical benefit remains to be substantiated.
Cup Capello et al., in a prospective study in which 514 cups were followed for three to five years, reported fewer radiolucent lines around hydroxyapatite-coated cups as compared with non-hydroxyapatite-coated cups of an identical design. Radiolucent lines in zone 3 were observed around 18.4% of the non-hydroxyapatite-coated cups but around none of the hydroxyapatite-coated cups. All revisions for aseptic loosening were performed for non-hydroxyapatite-coated cups. It is unclear whether hydroxyapatite coating will further improve the long-term durability of fixation of porous-coated cups. If it does, the issue of wear at the articulation will become even more important. Revision Surgery
Stem Modular stems are attractive for use in revision surgery because of their versatility in fulfilling the anatomical challenges created by bone defects. McCarthy et al. reported the ten-year results associated with the use of a modular stem. The design used in that study offered a proximal porous sleeve and a distal stem with flutes. Twenty-five (74%) of the thirty-four stems were bone-ingrown. Proximal bone preservation was evident in all twenty-five of these hips. Distal femoral hypertrophy was evident in 52% of the hips with bone ingrowth. All failures occurred in femora with segmental and diaphyseal bone deficiency, reflecting the limitations of this particular stem design. Hozack reported the short-term results associated with the use of contemporary modular revision stems in a study of ninety-five total hip arthroplasties. Sixty-four of the ninety-five patients had a Paprosky type-III or IV defect. Three stems were rerevised for loosening within two years. Subsidence of >10 mm was noted in association with fifteen stems, and subsidence occurred in 71% of the patients with type-IV defects. An extended femoral osteotomy appeared to enhance healing and stem fixation. These latest data on femoral revision surgery clearly underscore the need to develop newer implants and surgical techniques to address severe femoral bone defects. Extensively coated stems have been used frequently for revision total hip arthroplasty. Engh et al. 9 reported the mean 13.3-year results of twenty-six femoral revisions that had been performed with use of an extensively coated stem. Seventy-three percent of the stems were judged to be bone-ingrown. Three stems were revised because of loosening. The ten-year survival rate with revision as the end point was 89%. Paprosky and Sporer reported on their experience with the use of long (10-in [25.4-cm]) extensively coated stems to treat hips that had proximal femoral bone defects. Seventy-one hips were followed for a mean of 4.2 years. The extensively coated stems were not durable in hips with Paprosky type-IIIB and IV defects or hips in which the distal canal diameter was >19 mm. The authors recommended the use of a long-stem tapered design such as the Wagner stem design or impaction-grafting techniques for these severe cases. Despite these limitations, the technique of using an extensively coated stem remains one of the best solutions for some complex revision situations. Maloney et al. reported on the fracture of fifteen extensively coated stems. The patients varied in terms of age (range, forty-four to seventy-six years), weight (range, 65 to 109 kg), and activity level. The time-interval from the index total hip arthroplasty to the stem fracture ranged from two to five years. The stem diameters were small, ranging from 10.5 to 13.5 mm. The stems were well-fixed distally and had poor proximal bone support. This is similar to what was described over three decades ago in association with cemented stems (the so-called cantilever mechanism). A fatigue fracture pattern was observed in association with all retrieved stems. Surgeons should be aware of this complication, especially if the stem diameter is small and there is poor proximal bone support. Noble et al., in a cadaveric study, evaluated the rotational stability of revision stems with a diaphyseal fit. Two different stem designs (plasma-sprayed cylindrical stems and fluted cylindrical stems) and three depths of insertion (20 mm, 40 mm, and 60 mm) were studied. Resistance to torsion increased progressively in association with increasing depth of insertion. There was no difference between the two stem designs at any of the three insertion depths. The most important finding was that the actual length of bone-implant contact was, on the average, just slightly over 50% of the presumed values at all three depths. To achieve adequate stability, the authors therefore recommended a minimum diaphyseal length of 50 mm for distal fixation with use of these stem designs.
Cup Wear at the articulation and resultant osteolysis has become the most common mechanism of failure. Therefore, liner exchange is frequently required. Lachiewicz, Messick, and Soileau reviewed their experience with liner exchange in a study of thirty-five hips. The liners were impacted into the existing locking mechanisms without cement. The revised liners did not dislodge; however, 20% of the hips dislocated. Other surgeons, including us, have also observed a high rate of dislocation following liner exchange. It is advisable to consider postoperative bracing for these patients or increasing the femoral component head size (if the cup diameter allows it) to minimize dislocation. Blumenfeld and Bargar reported a 15% rate of dislocation following thirty-eight revision total hip arthroplasties that involved liner and head exchange. That study was especially important as the surgeons used a hooded liner in 61% of the hips, a head with an increased neck length in 55% of the hips, a head with a larger diameter in 45% of the hips, and a head with both an increased neck length and a larger diameter in 29% of the hips. Constrained liners have been used with increasing frequency to address the problem of recurrent dislocation; however, more data on the failure and limitations of these liners have been presented recently. Paprosky et al. reported a 19% rate of rerevision of constrained liners in a study of seventy-four total hip arthroplasties. The hips were separated into three categories: those with recurrent dislocation following primary total hip arthroplasty (twenty-three hips), those with recurrent dislocation following revision total hip arthroplasty (sixteen hips), and those with intraoperative instability during a revision procedure (thirty-five hips). Three different designs were used: SROM (sixteen hips), DePuy (twenty-three hips), and Osteonics (thirty-five hips). The rate of failure in the primary total hip arthroplasty group was 13%; all of these failures were due to malpositioning of the cup at the time of the index procedure. The rate of failure in the revision total hip arthroplasty group was 44%; all of these failures occurred in hips that had received the DePuy implant. The rate of failure was 11% in the intraoperative instability group; all of these failures occurred in hips in which the cup diameter was >62 mm. On the basis of that study, it can be concluded that malpositioning of the shell cannot be successfully addressed with a constrained liner, that some designs work better than others, and that constrained liners should be used with caution when jumbo cups are used. Complications The length of in situ service of total hip replacements has continued to increase with the improvements in implant design, fixation methods, and surgical techniques as well as the advances in the medical treatment of comorbidities that have resulted in a longer life expectancy for patients. Longer in situ service undoubtedly will result in a greater volume of complications, which not only compromise the functional outcome but also are costly. Minimizing the occurrence of complications and effectively managing them are currently among the most important clinical issues related to total hip arthroplasty.
Infection McPherson et al. 11 reviewed the results associated with the use of one such staging system in a study of fifty consecutive patients who had a chronic infection at the site of a total hip arthroplasty. The mean duration of follow-up was twenty-four months. Reimplantation of another total hip replacement was performed in 58% of the patients, permanent resection arthroplasty was performed in 34%, and amputation was performed in 8%. Ten percent of the patients died. All of the patients who died had advanced systemic comorbidities. The overall clinical satisfaction was marginal as evidenced by an improvement of the Harris hip score from 35 points preoperatively to 62 points at the time of the final follow-up. There was, however, greater improvement in the mean pain score, from 14 points preoperatively to 34 points at the time of the final follow-up. The presence of systemic comorbidities was strongly correlated with complications of treatment (p = 0.0003) and the rate of amputation (p = 0.014). The local wound condition was correlated with complications (p = 0.04) and the use of a muscle flap (p = 0.005). The local wound condition was not correlated with success of reimplantation, resection, or amputation. There was a positive correlation between advanced systemic and local comorbidities and the Harris pain score. Hanssen and Osmon 12 applied a similar staging system in the management of twenty-six consecutive patients. Reimplantation was performed in twenty patients, and permanent resection was performed in six. The reinfection rate among patients with a new total hip replacement was 20%. All of these patients had a massive structural femoral allograft. The reinfection rate was more than twofold higher in patients in whom the initial infection developed following a revision total hip arthroplasty (27%) than it was in patients in whom the infection developed following a primary total hip arthroplasty (12.5%). The failure rate was 22% for patients with methicillin-resistant organisms. There were no instances of reinfection among the six patients with a permanent resection. The numbers were too small for the authors to establish any correlation between the parameters of the staging system and outcome. Cierny and DiPasquale 13 described a treatment protocol on the basis of their experience with the use of a staging system in a study of forty-three patients. The most important parameter that was used to determine treatment was systemic comorbidity in the host. Reimplantation was done in 91% of the patients with minimal comorbidities (A-hosts) but in only 24% of those with multiple comorbidities (C-hosts). The two-year implant survival rate was 100% for A-hosts and 0% for C-hosts. Another very important finding was that 46% of the patients with late and refractory infections failed to have a positive culture of tissue obtained at the time of the index débridement procedure despite having had a positive culture of an aspirate before surgery. Moreover, 23% of the patients had new bacteria identified on culture of tissue obtained at the time of the second-stage surgery. The most widely used treatment protocol for infection at the site of a total hip arthroplasty is two-stage reimplantation. Wentworth et al. 14 reported on their experience with the use of an antibiotic-impregnated-cement articulating spacer (the PROSTALAC device) in 135 patients. Hybrid fixation was most frequently used for reimplantation. The rate of persistent infection or reinfection was 17.8% for hips in the PROSTALAC study, compared with 23.1% for a series of 134 hips that had been treated without the PROSTALAC spacer as reported in the literature from 1960 to 1998 (p = 0.40). Fewer implants required removal or revision in the PROSTALAC group compared with the control group (p = 0.02). Forty-nine complications occurred after the insertion of the PROSTALAC implant, and twenty-one complications occurred after the second-stage reimplantation of the permanent implant. Charlton et al. 15 also reported a high rate of complications in a study of forty-four hips that had reimplantation of a total hip replacement following a previous resection arthroplasty for infection. Cementless fixation was used in 84% of the hips. The most frequent complications were dislocation (five hips; 11.4%) and trochanteric nonunion (four hips; 9.1%). The mean Harris hip score improved from 40 points preoperatively to 78 points at the time of the final follow-up. Thirty-nine percent of the patients had a persistent limp, and 50% had persistent leg-length inequality. The rate of reinfection was only 2.3%. One of the more promising results was a high rate of stable implant fixation at two to nine years of follow-up, with 100% of the cups and 98% of the stems considered to be stable.
Dislocation The highest rate of dislocation following primary, uncomplicated total hip arthroplasty has been noted in association with the posterior surgical approach. Berry and Harmsen recently reported the dislocation rate following more than 22,000 total hip arthroplasties that had been performed over thirty years at the Mayo Clinic. The cumulative risk of any dislocation was 2.2% at one year, 3.8% at ten years, and 6% at twenty years. The ten-year risk was 3.2% for the anterolateral approach compared with 6.8% for the posterior approach (p < 0.0001). In a multivariate model, the relative risk of dislocation was significantly greater in association with 22-mm, 26-mm, and 28-mm femoral heads as compared with 32-mm heads, with p values ranging from 0.02 to <0.0001. The risk of dislocation associated with the posterior approach can be reduced with use of a capsular repair. Sioen et al. 17 recently confirmed its value in a cadaveric model. Those authors demonstrated a significant improvement in torsional strength to failure when a transosseous repair was compared with no repair (p = 0.0002) and with a soft-tissue repair alone (p = 0.002).
Thromboembolic Disease Surgical Techniques
Minimally Invasive Total Hip Arthroplasty The single-incision approach was originally proposed by Sculco from the Hospital for Special Surgery over five years ago. At the 2003 annual meeting of the American Academy of Orthopaedic Surgeons, Sculco and his colleagues presented the two to five-year follow-up results of a prospective, controlled study in which forty-two total hip arthroplasties that had been done with the MIS technique (mean incision length, 8.8 cm) were matched with forty-two total hip arthroplasties that had been done with a conventional technique (mean incision length, 23 cm). The operative time was shorter in the MIS group (p = 0.02). There was, however, no difference between the groups with regard to blood loss or length of stay. There were no instances of infection, nerve palsy, component malpositioning, or loosening in either group. There were no dislocations in the MIS group, compared with one dislocation in the conventional incision group. The postoperative Harris hip score was slightly higher in the MIS group (p = 0.04). The same group of surgeons also conducted a prospective, randomized study in which a single-incision MIS procedure performed through an 8-cm incision was compared with a standard procedure performed through a 15-cm incision. There were no differences between the groups with regard to demographic characteristics, operative time, wound drainage, length of hospital stay, or narcotic use. The MIS group had less intraoperative blood loss (p < 0.03), less total blood loss (p < 0.009), and a lower prevalence of limp at six weeks (p < 0.04). The senior surgeon advocated the use of this technique for patients with a body-mass index of <30. Other surgeons have utilized a similar surgical approach. Dorr reported on his experience with the use of a 6.5 to 11-cm incision in a study of 100 consecutive total hip arthroplasties. The incision required extension for exposure in two cases. There were no instances of dislocation, but there was one instance one each of infection and sciatic nerve palsy. Functional assessment demonstrated that the improvement in gait and muscle strength was faster than that for historical controls who had had a conventional incision. Goldstein presented the results of a study in which eighty-five total hip arthroplasties that had been performed with use of the MIS technique were compared with eighty-five total hip arthroplasties that had been done with use of a conventional incision. There were no differences between the groups with regard to demographic characteristics, operative time, or the need for transfusion. The MIS group had significantly less intraoperative blood loss (p < 0.001), but there was a selection bias for MIS in thinner patients as reflected by a significant difference between the groups with regard to the body-mass index (27 compared with 31; p < 0.001). The two-incision technique was first performed by Berger at Rush-Presbyterian-St. Luke's Hospital in 2000. To date, over 300 such procedures have been done at selected centers in the United States. Berger presented his own experience with the most recent eighty total hip arthroplasties that were performed after an initial "learning curve" period. The operative time ranged from eighty to 120 minutes. Berger reported no instances of infection, nerve palsy, dislocation, component malpositioning, or loosening. One femoral fracture occurred during surgery. All eighty patients were discharged to home either on the day of surgery or within twenty-three hours after surgery. Berger also reported no readmissions or postoperative complications. Keggi et al. presented their thirty-year experience with the use of a modified anterior approach. This technique utilizes the interval between tensor fasciae latae and the sartorius without disturbing the abductors. One or two additional incisions can be made to facilitate passage of the acetabular reamers and femoral broaches. The authors specifically reviewed the clinical data from 2132 primary total hip arthroplasties that had been performed between 1990 and 2000. Cement fixation had been used in 60% of the procedures. The prevalence of dislocation was 0.8%, with only one case requiring revison. The prevalence of hematoma was 1.4%, and there were six deep infections. The most common complication was fracture of the proximal part of the femur during broaching (prevalence, 4.1%). Cerclage wire fixation was needed for the treatment of 24% of these fractures (1% of the entire series). Symptomatic thromboembolism occurred in only seventeen patients (0.8%). Surgical navigation systems may be especially useful in minimally invasive surgery. DiGioia et al. 20 conducted a prospective study in which patients who had been treated with the MIS technique with the assistance of a navigation system were compared with patients who had undergone a conventional arthroplasty through a posterior approach. The MIS group had a lower prevalence of limp (p < 0.05) and a greater ability to climb stairs (p < 0.01) at three and six months of follow-up. Moreover, patients in the MIS group were able to walk longer distances at six months (p < 0.001). There was no difference between the groups at one year after surgery. Cup placement in the MIS group was very satisfactory with the use of navigation system: the mean abduction was 46°, and the mean anteversion was 22°. Minimally invasive surgery may be useful for selected patients, especially if the surgeon has had adequate training and has gone through the necessary learning curve. Special retractors and navigation systems may be of additional benefit to the surgeon. Moreover, different instruments, retractors, and even implant designs (such as modular components) may further enhance the reproducibility and clinical efficacy of minimal incision total hip arthroplasties. At the present time, there are no scientific data supporting the clinical superiority of MIS beyond the short-term follow-up data reported here. Complications and implant fixation durability must be carefully tracked over the long term. Articulation and Wear
Activity Level and Wear
Polyethylene Oxidative degradation has long been recognized as being detrimental to polyethylene. Hopper et al. 22 measured the wear rates associated with two types of conventional polyethylene liners of the same design. Only the sterilization technique was different: one group of liners had been sterilized with gamma radiation in air, whereas the other group had been sterilized with gas plasma. Liners that had been sterilized with gamma radiation had significantly lower wear rates (p < 0.001). Sterilization technique (p < 0.001) and age at the time of surgery (p = 0.001) were the only variables that were found to correlate with the wear rate. The same group of investigators analyzed the wear rates following 261 total hip arthroplasties that had been performed with use of an identical cup design. They found poor correlation between early and late wear rates (r 2 = 0.11). This finding underscores our limited ability to predict late wear rates on the basis of early radiographic measurements. It is therefore important to continue the surveillance of total hip arthroplasties with regularly scheduled radiographic evaluations. Accurate measurement of polyethylene wear in vivo is of paramount importance when monitoring patients and when evaluating the performance of different polyethylene products after in situ service. Two computerized radiographic wear-analysis programs have been developed and are in use: Devane's technique (PolyWare) and Martell's technique (Hip Analysis Suite). Hui et al. 23 conducted a study to validate these two techniques. Twenty-seven liners were retrieved at the time of revision surgery after a mean of 9.2 years in situ. All liners were of the same design, and all had been sterilized with gamma radiation in air. There was good correlation between the two computerized techniques and an established coordinate measuring technique in terms of both two and three-dimensional linear penetration values (r 2 = 0.78 to 0.91; p < 0.001). However, both computerized techniques underestimated true wear as measured with the coordinate measuring machine. Multiple factors can influence the radiographic measurement of polyethylene wear, including the quality of the radiographs, bedding-in, and the manufacturing tolerances of the shell and the liner. These techniques are accurate to a variance in the range of 0.25 to 0.41 mm. Additional study is needed to substantiate the accuracy of these techniques for the evaluation of liners with less polyethylene wear and those in patients with well-functioning total hip arthroplasties.
Highly Cross-Linked Polyethylene
Ceramic Ceramic-on-ceramic couplings have continued to receive increasing attention as an alternative to metal-on-polyethylene couplings. D'Antonio et al. reported no difference in terms of clinical and radiographic results, after three to five years of follow-up, in a prospective, randomized clinical trial in which ceramic-on-ceramic couplings were compared with metal-on-polyethylene couplings that were used with similar cup and stem designs. To date, the same surgeons have implanted 947 ceramic-on-ceramic couplings of this particular design without encountering a ceramic bearing fracture. Sedel reported thirteen component fractures (eight head fractures and five cup fractures) among 5500 alumina ceramic components that had been implanted over twenty-five years. The component fractures occurred between three months and twenty-three years after implantation. Eight of the thirteen fractures were attributed to trauma or poor design, and two were attributed to early-generation material; no cause could be identified in three hips. It is hoped that this rare but catastrophic complication will not occur in association with newer alumina ceramic material and design features.
Monitoring of Pelvic Osteolysis Claus et al. conducted two separate cadaveric studies to evaluate the value and accuracy of identifying and quantifying pelvic osteolysis with use of radiographs and computerized tomographic scans. Defects of predetermined size were created in the acetabulum. The overall sensitivity of detecting a defect on an anteroposterior, obturator oblique, or iliac oblique radiograph was only 41.5%, and the overall specificity was 93%. Sensitivity was highly dependent on the size of the lesion being >10 cm 3 (p < 0.001). Sensitivity was also dependent on the location of the defect; specifically, the sensitivity was 72% for lesions in the ilium, 31% for lesions in the pubis, and <15% for lesions in the ischium. The size of the defect as estimated by the surgeon on the basis of the radiographs was, on the average, twofold smaller than the actual size. In contrast, while computerized tomographic scans overestimated the volume of the defect by a mean of 5.6%, the sensitivity improved to 81%. Most importantly, the sensitivity was greatly improved for the detection of defects in the ischium (78%) and in the posterior acetabular rim (89%). These laboratory data underscore the importance of developing better techniques to detect and to monitor pelvic osteolysis. Growth Factors and Biology
Growth Factors Two groups of investigators have used growth factors to enhance bone-graft incorporation in a canine total hip arthroplasty model. Barrack et al. created a defect in the weight-bearing region of the acetabulum and filled it with recombinant human osteogenic protein-1 (OP-1), allograft, or no bone graft. A porous-coated cup was inserted against the defect. The authors demonstrated that defects that had been treated with OP-1 healed more completely and achieved a bone density that was equivalent to that in animals without a defect. The quantity of bone growth into the porous coating from the defect was equivalent to that from an intact acetabulum. Bragdon et al. created a peripheral acetabular defect and filled it with recombinant human bone morphogenetic protein-2 (rhBMP-2). There was consistent bridging of the defects and bone growth into the porous surface of the cup in animals treated with rhBMP-2 that did not occur in the control animals. These growth factors are promising adjuncts to allograft alone in the treatment of acetabular bone defects. Moreover, they appear to potentiate bone ingrowth on porous surfaces.
Biology of Osteolysis Tumor necrosis factor-alpha (TNF-alpha) is a cytokine that is released in response to phagocytosis of wear debris. It stimulates precursor cells to differentiate into osteoclasts. Anti-TNF-alpha drugs have been used in the clinical treatment of rheumatoid arthritis. Childs et al. 26 used an adenovirus vector to induce anti-TNF-alpha to inhibit osteolysis in a mouse model. The authors also reported, however, that the vector itself caused an immune response. Thus, adenovirus probably is not an ideal vector for gene therapy in the treatment of osteolysis. Other scientists have reported successful transduction of a regulatory protein in the pathogenesis of osteolysis without any adverse effects in a mouse model. These representative studies provide hope that gene therapy may be a viable treatment for osteolysis in the future. Outcome Analysis and Practice Management
Epidemiology and Adverse Outcomes It is critical to analyze the outcome of total hip arthroplasty in the wider community orthopaedic practice rather than at selected tertiary referral centers. Analyses involving the use of a large database such as the Medicare database have been done over the last decade. Mahomed et al. 27 analyzed the Medicare database to determine the rates and outcomes of primary and revision total hip arthroplasties. Information was extracted from a cohort of more than 61,000 primary and 13,000 revision total hip arthroplasties. The rate of primary total hip arthroplasty ranged from 17.6 to 29.5 per 10,000 patients, and the rate of revision total hip arthroplasty ranged from 3.9 to 7.6 per 10,000 patients. These rates increased with increasing age until the age of seventy-five to seventy-nine years and then declined. The ninety-day mortality rate was 1.0% for primary total hip arthroplasty and 2.6% for revision total hip arthroplasty. The readmission rate was 4.6% for primary total hip arthroplasty and 10.0% for revision total hip arthroplasty. The rate of complications was generally higher after revision total hip arthroplasty. Factors associated with an increased risk of an adverse outcome included increasing age, male gender, African-American racial background, medical comorbidity, and lower income. Using the same database, Phillips et al. 28 analyzed the prevalence of three specific complications (dislocation, pulmonary embolism, and infection) following total hip arthroplasty. The incidences of these complications in the first six months following primary total hip arthroplasty were 3.9%, 0.9%, and 0.2%, respectively. The incidences following revision total hip arthroplasty were 14.4%, 0.8%, and 1.1%, respectively. These adverse outcomes were more frequent during the acute hospitalization, and they diminished in the months after surgery. However, they continued to occur throughout the first three months after surgery, and it is of concern that some of these complications will occur in the outpatient setting given the continuing trend to shorten hospital stay. It is hoped that these data will serve as a reference for the development of preoperative protocols to optimize patient risks as well as postoperative rehabilitation and monitoring protocols to minimize complications. Mears et al. specifically investigated whether a shortened length of hospital stay resulted in a greater complication rate following total hip arthroplasty. The authors reviewed 100 consecutive total hip arthroplasties that had been done in 1987 and compared them with a matched group of 100 total hip arthroplasties that had been done in 1997. The two groups were matched for risk factors according to the American Society of Anesthesiologists (ASA) classification system. The mean length of stay was 13.4 days in 1987 and 5.1 days in 1997. The rate of major complications was 11% in 1987 and 4% in 1997 (p = 0.107). Cost-reduction strategies such as early discharge did not appear to adversely affect the short-term outcome of total hip arthroplasty at that particular institution.
Timing of Total Hip Arthroplasty
Clinical Outcome Measurement Lieberman et al. 31 reported intriguing data with regard to patient expectations related to total hip arthroplasty. The authors followed 101 patients before and after total hip arthroplasty with use of the McMaster Toronto Arthritis Patient Preference Disability (MACTAR) instrument, a validated twenty-five-item questionnaire, to measure patient expectations regarding the procedure. Expectations of improvement in pain and walking distance were the most consistent before and after surgery. Only 41% of the patients chose three or more of the same items among their top five before and after surgery. Twenty-nine percent of the patients actually identified three or more new items after surgery. It is thus important for the surgeon to be aware that patient expectations may change after surgery. This variance may account for some of the suboptimal results as measured with other validated outcome instruments. It is intuitive that patients with greater physical fitness will have a faster rehabilitation after major surgery. Gilbey et al. 32 conducted a prospective, randomized study to compare the functional recovery in a group of patients who underwent an exercise program before and after total hip arthroplasty (the exercise group) with that in a group of patients who had routine physical therapy after surgery (the control group). The patients in the exercise group completed an eight-week exercise program before surgery. Both groups were tested at baseline, at one week before surgery, and at three, twelve, and twenty-four weeks after surgery. There was no difference between the groups at baseline. Patients in the exercise group had significantly greater improvement in terms of the WOMAC score (p = 0.02), stiffness (p = 0.01), strength (p = 0.04), and physical function (p = 0.04) than did those in the control group. There was, however, no difference between the groups in terms of pain perception. These differences continued from Week 3 to Week 24 after surgery. These data reflected not only improvement just before total hip arthroplasty but also an increased rate of improvement after surgery in the exercise group. Exercise may be a very cost-effective intervention to reduce the length of in-patient stay (acute and subacute care) and the costs of postoperative rehabilitation.
Pain Management
Transfusion Management
One-Stage Bilateral Total Hip Arthroplasty Kim et al. 35 conducted a prospective, randomized study in which the prevalence of fat embolism following one-stage bilateral total hip arthroplasty was compared with that after unilateral total hip arthroplasty. Fifty patients underwent a one-stage bilateral procedure, and 100 patients underwent a unilateral procedure. Approximately 50% of the stems were inserted with cement. Multiple blood samples were obtained for analysis from peripheral arterial and right atrial lines. There was no difference in the prevalence of fat or bone-marrow-cell embolism between the groups treated with bilateral and unilateral total hip arthroplasty or between the groups treated with and without cement. Educational Opportunities Members of The Hip Society actively participate in educational programs in conjunction with the American Academy of Orthopaedic Surgeons and the American Association of Hip and Knee Surgeons. The American Academy of Orthopaedic Surgeons generally sponsors two or three instructional courses focusing on total hip arthroplasty issues each year. The annual meeting of the American Association of Hip and Knee Surgeons is routinely held in Dallas during the first weekend in November each year. The Hip Society Winter Meeting is routinely held on Specialty Day during the annual meeting of the American Academy of Orthopaedic Surgeons. These meetings provide an excellent means of keeping abreast of the dynamic field of adult reconstructive hip surgery.
References
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