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© 2000 The Journal of Bone and Joint Surgery, Inc.
Patellar Malalignment*
*No benefits in any form have been received or will be received
from a commercial party related directly or indirectly to the subject
of this article. No funds were received in support of this study.
 Patellar malalignment is underdiagnosed yet too frequently
treated with surgery.
Patellar malalignment is a translational or rotational deviation of the patella relative to any axis, and it can be a major component of patellar pain in adults. Chondromalacia, the word that has defined the field of patellar pain during the last century5,14,92, is now mostly a source of confusion and should be abandoned19. Indeed, the presence of soft cartilage - the literal translation of the term chondromalacia - is not strongly correlated with patellar pain1,28,175. Moreover, surgeons often refer to cartilage changes anywhere in the knee as chondromalacia. The confusion is further compounded by the use of poorly defined grades (for example, grade-II chondromalacia). Lesions are best called chondral or cartilage lesions, and they should be described qualitatively (for example, as blistering or fissuring) and quantitatively (in terms of size, location, and depth). The term patellar subluxation is also confusing, as it can pertain to a symptom, a finding on the physical examination, or a radiographic observation75. There is no universally accepted understanding of the term, and it requires an explanation with each use.
Patients with patellar symptoms can be divided into two groups: those who feel the patella slip, and those who simply have pain. This slipping, referred to as instability or subluxation, has a mechanical basis. Several mechanical causes have been proposed. The vastus medialis obliquus may be proximally positioned and abnormally oriented, and the timing of its contractions may be poorly synchronized with those of the surrounding muscles96. Osteochondral dysplasias, such as a shallow or even a convex trochlear groove, may be present38,155. The patella may lie too far proximally or distally relative to the trochlea, conditions called patella alta and patella baja (or patella infera), respectively22. Finally, the patient may exhibit generalized ligamentous laxity or complex malalignments of the entire extremity181. Pure patellar pain remains more poorly understood. Chondral lesions themselves are asymptomatic49, although cartilage that is worn down to bone remains a potential source of pain. Ficat et al.55 subscribed to the concept of patellar malalignment as a source of pain. Specifically, they hypothesized that if the patella is tilted, lateral side down, painful stresses can develop on its lateral aspect. This is still an accepted theory19,51, although not all tilted patellae are painful. Pain from patellar malalignment appears to be related to multiple factors with variable clinical expression45, and our imperfect understanding of these factors may explain the all-too-frequent failure to achieve adequate pain relief with the use of realignment procedures. Malalignment appears to be a necessary but not sufficient condition for the onset of pain. Symptoms seem to be set off by a trigger, the nature of which varies from patient to patient. Trauma is a potential trigger6,19,84,133,163,179. The presence of nerve-fiber changes in the lateral retinaculum61,64 raises the possibility that these changes might be a catalyst for pain, although it is not clear whether the malalignment precedes the nerve changes or vice versa. There are reports of familial malalignment161, which lend credence to the concept of malalignment as a possibly inheritable trait. Certain disorders of the patella and the extensor mechanism can be associated with patellar malalignment. These disorders include neural inflammation61,163, altered proprioception69,104, venous congestion9, Osgood-Schlatter disease109,130, bipartite patella101,147,165, and inflammation of the patellar tendon (so-called patellar tendinitis)46,60. There are also causes of patellar pain unrelated to malalignment, such as overuse50,131, inflammatory arthritis, chemical irritation187, osteochondritis dissecans144, tumor53, plicae44, infection, stress fracture149, reflex sympathetic dystrophy78, fat-pad inflammation47, dorsal defects42,175, referred pain from another site, and emotional disturbances.
In view of the frequent problems associated with the knee, it is remarkable that much of the time the articulating surfaces are not in contact. There is no patellofemoral contact in the early degrees of knee flexion and thus no contact during standing or walking on level ground. Perhaps this is why symptomatic malalignment is not even more prevalent than it is. At any given time, only a portion of the patellar articular cartilage is in contact with the trochlea. As the knee flexes from 10 to 90 degrees, the contact area shifts gradually from the distal to the proximal pole of the patella67. As the knee flexes toward 120 degrees, the contact area moves back toward the center of the patella. With flexion of greater than 120 degrees, there is no contact between the patella and the medial femoral condyle, except for that provided by the small, nearly vertical, odd facet, which is present in 80 percent of people78,111. In a closed-chain activity, where the leg, foot, and ground form a continuous kinetic chain, the articular pressure increases as the knee flexes from 0 to 90 degrees78. Indeed, the joint-reaction force increases proportionately more than the magnitude of the contact area78. From 90 to 120 degrees, the articular pressure remains essentially unchanged because the quadriceps tendon comes into contact with the trochlea and effectively increases the contact area.
The articular cartilage of the patella is the thickest in the
body (up to seven millimeters thick), and it is unique in its lack
of conformity with the underlying bone. Thus, the cartilaginous
apex of the patella, when viewed in the axial plane, rarely coincides
with the osseous apex172. The
same is true for the trochlea, but to a lesser extent. The nature
and number of articular facets appear to be unique to each individual111. The biomechanical properties of
the patellar cartilage have been found to differ from those of the trochlea134,135. Specifically, the patellar
cartilage has a greater permeability and a lower compressive aggregate modulus.
For isotropic materials, Young's modulus E, Poisson's ratio Through most of the flexion-extension cycle, the force generated by the patellar tendon is lower than that generated by the quadriceps tendon4,17,25,95. Thus, force is dissipated as it rounds the patella. As with certain other levers in the human body, such as the shoulder and the elbow joint, force is sacrificed for displacement (a lever amplifies force at the expense of displacement, and vice versa)136. The insertion site of the normal vastus medialis obliquus is the medial border of the patella, between one-third to one-half of the way down from the proximal pole. The vector of the muscle is medially directed and forms an angle of 50 to 65 degrees with the mechanical axis of the leg21,108,159. In patients with malalignment, the vastus medialis obliquus may not reach the patella, and its most distal portion may remain proximal to the proximal pole of the patella. The muscle vector is then more vertical than normal, making it less effective as a dynamic stabilizer64,78. The timing of the vastus medialis obliquus contractions relative to those of other muscles, especially the vastus lateralis, is critical and can be abnormal in the setting of malalignment117. A tight lateral retinaculum pulls the patella laterally and tilts it, lateral side down85,128. On the medial aspect of the patella, the medial patellofemoral ligament, which originates from the adductor tubercle and inserts on the proximal two-thirds of the medial aspect of the patella, appears to be the strongest passive, medial restraint33,85. It can be stretched in the face of chronic malalignment, and, in an acute dislocation, it can be torn from its femoral origin or patellar insertion. Patellar tracking continues to be the subject of much study88,183. Patellae take a serpentine, individualistic path when assessed relative to coordinates that are termed global when they are centered on a testing jig183. However, when normal patellae are investigated with use of anatomy-based coordinates, they behave in a similar manner, assuming a slightly lateralized position in neutral (0-degree) extension and taking a straight path down the trochlea as the knee is slightly flexed10,78.
Karlson105, in 1940, noted that "the diagnosis is difficult to make and the differential diagnosis of injury to the meniscus . . . causes special difficulties, as in both these ailments [meniscal and patellar pathology] there is a pressure tenderness over the medial joint space." Hughston et al.96 echoed those sentiments in 1960 and then in 1984, when they stated that "the orthopaedic surgeon who has not mistaken a recurrent subluxation of the patella for a torn meniscus has undoubtedly had a very limited and fortunate experience with knees and meniscectomies." The problem is now magnified because of the relative ease of performing a meniscectomy. Thus, arguably, the main problem related to the management of patellar malalignment is not the treatment itself but recognition of the condition. The symptom of giving-way should arouse suspicion of patellar malalignment, although this symptom is shared by other conditions. Pain occurs when a patient arises from a seated position, climbs stairs, or squats, as these activities exacerbate the abnormal pressure distribution about the patella. The so-called movie-theater sign96 - pain with prolonged sitting - traditionally has been associated with patellar pain from any source. Venous congestion and stretching of painful tissues are potential explanations for this symptom. With regard to the diagnosis of patellar malalignment, the location of the pain is not as helpful as one would expect; classically, the pain is anterior, but it can also be medial96,105, lateral96,105, or popliteal99. Unrelenting pain should arouse suspicion of a neurological condition, such as a neuroma132,156, reflex sympathetic dystrophy, or radicular pain, especially if the pain is burning in nature. A number of findings on the physical examination have been correlated with patellar symptoms, but there is no one sign that is currently accepted as proof of malalignment157. The patient should first be examined in the standing position to assess overall limb alignment as well as to identify gait and foot abnormalities such as foot pronation. Then, while seated, the patient should fully extend the knee from a flexed position. A J sign is considered to be present when the patella assumes a lateralized position as the knee approaches full extension. The patella is compressed with the palm of the hand through the full range of motion, as ulcerated lesions can be tender with this provocative test. Distal lesions are tender in the early degrees of knee flexion, whereas proximal lesions are tender at approximately 90 degrees. Crepitus is a nonspecific finding and can be associated with both cartilaginous and synovial lesions. With the patient supine and then prone, the hip is flexed and rotated as the surgeon checks for a source of referred pain and assesses anteversion96. The skin, retinacula, quadriceps tendon, and patellar tendon are palpated to rule out soft-tissue sources of pain. In patients with malalignment, the vastus medialis obliquus typically cannot be identified by sight or palpation; it inserts no farther distally than the proximal pole of the patella, and it remains soft, even with maximal quadriceps contraction96. With the knee extended, tilt is assessed by lightly palpating the medial and lateral borders of the patella. The facets, with intervening soft tissues, can be tested for tenderness, and overhang of the lateral facet over the trochlea can occasionally be assessed. The quadriceps (Q) angle is a measure of the patella's tendency to move laterally when the quadriceps muscles are contracted70,96. This is defined as the acute angle formed by an imaginary line connecting the anterior superior iliac spine to the center of the patella and a line connecting the center of the patella to the tibial tuberosity. The Q angle is increased in patients with a lateralized tibial tuberosity, but it can be falsely normal when the patella is laterally displaced. Different values for the normal ranges of the Q angle have been reported in the literature, and slight differences are noted when men are compared with women189, when the patient is assessed standing189, and when the quadriceps are contracted. With the knee slightly flexed, a Q angle of greater than 20 degrees is considered to be abnormal96,186,189. The Q angle is increased in only a small percentage of patients with patellar pain; as with other clinical signs, an abnormal value does not necessarily identify the source of pain nor should it be considered an indication for operative treatment157. Although increased Q angles are traditionally associated with valgus knees, some of the highest Q angles are found in patients with a combination of genu varum and proximal tibial torsion82,96. Patellar malalignment is associated with tightness of the following structures (in decreasing order of frequency): the lateral retinaculum, the hamstrings, the iliotibial band, the quadriceps, the hip rotators, and the Achilles tendon64,96. The lateral retinaculum is assessed by way of patellar tilt and medial-lateral displacement (glide)58,107,117,177. The Ober test is used to evaluate tightness of the iliotibial band78. For this test, the patient is placed in the lateral decubitus position, and, with the hip extended and abducted and the knee flexed, the proximal part of the leg is allowed to drop passively onto the contralateral limb. The test is considered positive when the leg fails to drop. Quadriceps tightness is examined by placing the patient prone and passively bringing the heels toward the buttocks. The heels should touch the buttocks. The apprehension sign, whereby the patient becomes apprehensive as the knee is extended while a laterally directed force is applied to the patella, is a reflection of extreme malalignment and is therefore an insensitive screening test for malalignment78.
Imaging can confirm a diagnosis of malalignment while further qualifying and quantifying it; however, it does not automatically identify the source of pain. A major advance has been the improvement of conventional radiography performed in an office setting. Few patients, therefore, require magnetic resonance imaging. The lateral radiograph reveals whether patella alta or baja (infera) is present. Although each has slight advantages and disadvantages, there are a number of simple parameters for the detection of patellar height16,30,37,72,98. Variations in the sagittal shape of the patella can affect these measurements74. If, on the lateral radiograph, the posteriormost portions of the femoral condyles overlap precisely, much information can be obtained with regard to both the trochlea and the patella (Fig. 1); for example, patellar tilt can be appreciated. Indeed, the osseous median ridge of the patella and the radiographic condensation of the lateral patellar border become confluent as the patella tilts122,137. This sign is particularly useful when tilt is present near extension but not in flexion. In this situation, tilt cannot be detected on the Merchant radiograph, since this image is made with the knee flexed at least 30 degrees, but it can be detected on lateral radiographs made with the knee extended. Moreover, when only the most proximal part of the trochlea is dysplastic, the dysplasia is best noted on the lateral radiograph because this portion of the trochlea is not visualized on a Merchant radiograph38,122,137. The difference in height between the osseous trochlea and the condyles is a measure of the depth of the trochlea. An intersection of the lines therefore indicates an absence of depth - that is, dysplasia71,122,155.
Although many tangential views, also known as sunrise views, have been tried over the last century, the Merchant radiograph127 has emerged as the standard, as it allows for imaging of the trochlea at 30 degrees of flexion rather than the common 60 degrees. Also, it does not artificially compress the patella as do radiographs made with the patient prone. The patient lies supine, with the calves and feet supported by a table extension. The degree of flexion is important because malalignment is more noticeable and more clinically relevant at 30 degrees of flexion than it is at 60 degrees. Two measures of malalignment are assessed on the axial radiograph: medial-lateral displacement177 and tilt73. These two parameters are independent of each other, although they coexist in the most extreme cases. Lateral displacement can be measured with use of the congruence angle127, which is normally negative3. Tilt can be measured with use of a number of references, including the (osseous) lateral facet and the anterior aspect of the femoral condyles164. However, the condyles can be dysplastic in the very patients who are likely to have malalignment; therefore, they are not ideal references. I prefer a technique that reflects the assessment of tilt on the physical examination: the drawing of a line from the medial to the lateral border of the patella as referenced from any horizontal line drawn on the radiograph73 (Fig. 2). The drawbacks to this approach are minor: the patient's legs must rest in neutral rotation, and the lower border of the x-ray cassette must be parallel to the ground. Normal tilt ranges from 0 to 5 degrees, and tilt angles of greater than 10 degrees are abnormal69.
Computerized Imaging Magnetic resonance imaging provides static pictures of a relaxed extensor mechanism in a supine patient. This is a questionable substitute for the imaging of a standing, active person. Moreover, magnetic resonance imaging does not effectively detect all chondral lesions. Nevertheless, because of the difference between the osseous and cartilaginous contours of the patella, magnetic resonance images give a more accurate picture of patellofemoral congruence than do radiographs. With regard to the detection of cartilage lesions, major advances continue to be made43,115,139,145,154,160. This is critical, since these lesions can be sources of pain and need to be considered in any operative planning. A laterally positioned patella can be found in asymptomatic subjects when their knees are imaged in full or neutral (0-degree) extension39. However, on routine magnetic resonance imaging, when the knee is flexed approximately 10 degrees, the normal patella is centered over the underlying femur77. Because the quadriceps are relaxed, magnetic resonance imaging can underestimate tilt and lateral displacement79. Computerized tomography can be used to superimpose images, as when the position of the tibial tuberosity is assessed relative to the trochlear groove68. Tilting of the patella has been evaluated. With tilt defined as the angle subtended by a line drawn along the posterior femoral condyles (or a line parallel to it) and by a line along the osseous lateral facet of the patella (Fig. 3), angles of greater than 7 degrees were found to be normal (at 15 degrees of knee flexion)64.
Nuclear Imaging Technetium scanning is the only test that provides insight into bone metabolism and homeostasis. In some patients with patellar pain, increased activity has been noted about the patella48. A positive scan does not differentiate between the possible causes of patellar pain, but it can validate a patient's complaints.
Patients with inflammatory arthritis may present initially with patellar pain, and the arthrosis can at first be localized to the patellofemoral joint. This is of particular interest if a patellectomy or a patellofemoral replacement is contemplated. The serum evaluation should include tests for the uric acid level, erythrocyte sedimentation rate, rheumatoid factor, and antinuclear antibodies. Consideration can also be given to testing for the Lyme disease titer, the HLA-B27 genetic marker, and tuberculosis.
Pain from malalignment is likely to persist or recur unless the subject modifies his or her activities. On the other hand, the natural history of a dislocation resulting from malalignment depends on the extent of the malalignment and the age of the patient27,35,120. The greater the underlying malalignment, the greater the tendency of the dislocation to recur86. Also, the younger the age at the time of the initial dislocation, the greater the predisposition to recurrent instability27. Over time, recurrent dislocations tend to decrease in frequency27,35, but each episode is still very painful and a potential source of chondral damage. A patellar dislocation mandates that a radiograph and possibly a magnetic resonance imaging scan be made. Arthroscopy and realignment procedures are not essential after an initial dislocation but are reasonable options when there is a suspicion of loose bodies or severe malalignment. Arthritis is a long-term risk with or without an operation7,102,121. It remains unclear to what extent a child's trochlea can remodel following a realignment procedure.
Most patellar pain diminishes with nonoperative treatment89. Patellar pain resulting from overuse will most likely decrease with rest, anti-inflammatory medications, and standard physical therapy regimens. Patients with malalignment require a more patella-specific approach78, since commonly used exercises can aggravate the pain and standard knee protocols fail to specifically address the malalignment. Stretching is the hallmark of a patellar malalignment rehabilitation protocol, which includes stretching of the lateral retinaculum, the iliotibial band, the hamstring muscles, the quadriceps, and the Achilles tendon117,186. Strengthening of the vastus medialis obliquus - the main dynamic stabilizer of the knee - is one of the main goals of patellar rehabilitation, but it remains unclear to what extent there are specific and preferential exercises for this muscle. As a rule, strengthening of the quadriceps is beneficial as long as it does not cause pain. There is controversy with regard to whether the exercises should be performed in an open or closed-chain manner173. Alternating between the two can sometimes allow pain-free training throughout the full arc of knee motion. Exercises can be modified according to the location of cartilage lesions; early flexion activities are discouraged when patellar lesions are distal, and exercises at approximately 90 degrees of flexion are avoided in the presence of proximal lesions. Coordination of muscle activity is as important as strength-training. The interplay between the vastus medialis obliquus and the vastus lateralis continues to be of great interest, and biofeedback with surface electrodes has been used as an adjunct to therapy78,106,188. Braces are prescribed to modify the position of the patella. These devices range from simple straps across the patellar tendon to complex supports. Although they relieve symptoms in a number of patients, their mode of action remains speculative and their effectiveness is unpredictable158,169. The goal of taping116,117,158 is to pull the patella away from a painful area, thereby unloading it and providing pain relief. The extent to which this is possible and the amount of displacement required to provide pain relief varies from patient to patient. Displacement need not be perceptible to effect improvement. A secondary goal is to inhibit muscle-firing by taping perpendicular to the direction of the muscle fibers124,125. For example, taping across an overly powerful vastus lateralis can diminish its pull116,117. Knee mechanics are affected by the position of the foot during gait; in certain patients, some aspects of patellar pain can be attributed to an abnormal foot. For example, foot pronation imparts internal torsion to the tibia and a valgus moment at the knee. It is therefore reasonable, for selected patients, to prescribe an orthotic device for the shoe78,153.
When clinical improvement with nonoperative treatment has ceased, operative intervention may be required. These operations can be divided into three categories: proximal realignment, distal realignment, and operations that are not intended to change the alignment of the extensor mechanism. Proximal realignment: These procedures alter the medial-lateral position of the patella through manipulation of the soft tissues proximal to its inferior pole. Included in this category are lateral retinacular release36,56,90,107,124,128,148, medial plication96,100,190, vastus medialis obliquus advancement115, and medial patellofemoral ligament repair and reconstruction66,85. Distal realignment: These operations modify the medial-lateral, anterior-posterior, and proximal-distal31,170 positions of the patella by way of transfers of the tibial tuberosity or through surgery on the soft tissues distal to the inferior pole. They include medial transfer of the patellar tendon162; medial transfer of the lateral portion of the patellar tendon65; anterior displacement of the tibial tuberosity52,54,103,113,123,151,166; medial displacement of the tibial tuberosity34,180; anteromedial displacement of the tibial tuberosity15,62,174; distal, medial, and posterior transfer of the tibial tuberosity83; semitendinosus transfer13; plication of the patellar tendon23; and miscellaneous osteotomies of the tibial tuberosity114. Operations not intended to change the alignment of the extensor mechanism: These procedures include arthroscopic synovectomy and débridement59; denervation of the patella40,94; cartilage146, periosteal93, and osteochondral grafting11; chondrocyte culture and transplantation146; microfracture146; patellar osteotomy117,140,184; prosthetic replacement8,18,26,76,81,129; and patellectomy2,29,112,191. It remains to be determined whether the surgeon should identify the pathoanatomy of each patient and plan operative treatment individually63 or whether there are one or more procedures that will be satisfactory for most patients. Some surgeons have performed distal realignment for instability regardless of the pathology and have reported success with this approach57,138, while others have achieved the same results with proximal realignment100. No long-term and few medium-term results have been reported for the more common procedures, to my knowledge. The trend is toward recreating the normal anatomy and thus toward customizing the procedure to the patient's specific pathoanatomy168. No realignment procedures, including lateral retinacular release, are indicated for patients with poorly defined pain, as such procedures may lead to malalignment97. Moreover, even a so-called minor procedure such as lateral retinacular release is associated with a definite rate of complications such as hematoma167. Operations that are rarely performed any longer include the Hauser procedure83, in which the tibial tuberosity is transferred distally, medially, and posteriorly. Late results of this operation have included a high rate of patellofemoral arthritis, presumably because of the excessive tightening of the extensor mechanism80,141. It is the posterior-displacement component of the transfer that is currently thought to be a problem. The Goldthwait operation65, in which the lateral portion of the patellar tendon is transferred medially under the remaining tendon, successfully diminishes the lateral translation of the patella but at the potential expense of increasing patellar tilt (lateral side down)178. For any operation involving a transfer of the tibial tuberosity, the tendency has been to include a longer rather than a shorter portion of the tuberosity151. This minimizes the risk of splintering the bone and losing fixation. In the case of an anterior displacement procedure, the longer tuberosity piece minimizes the acuteness of the angle between it and the tibial shaft, leading to a better cosmetic result and perhaps to less risk of tendinitis. Some investigators have recommended rotation of the entire tibia rather than transfer of the tuberosity126. In the patient with open epiphyses, it is prudent to approach tibial tuberosity procedures with caution in order to avoid creating a growth arrest at the apophysis and eventual recurvatum. Soft-tissue procedures about the tuberosity are an acceptable alternative20. The greatest challenges involve patients with major trochlear dysplasia and those with arthritic lesions localized to the patella or the trochlea. When the proximal surface of the trochlea is convex rather than concave, realignment procedures are usually unsuccessful. The patella will continue to subluxate either laterally or medially, and attempts to tighten the extensor mechanism may lead to arthritis. It is important to detect these dysplasias preoperatively, both to construct a realistic operative plan - an isolated lateral release, for example, will not work - and to provide the patient with adequate data on which to base informed consent. Attempts to change the shape of the trochlea have been disappointing125, although research in this area continues. When dealing with arthritic lesions, there are a number of options. Stresses on the lesions can be minimized by displacement of the patella; fresh cartilage can be implanted through chondral and osteochondral transplantation as well as through cartilage culture followed by reimplantation146; the subchondral bone of the arthritic bed can be drilled or fractured to stimulate the formation of cartilage146; resurfacing can be performed with a patellar or trochlear replacement26; or a partial or total patellectomy can be carried out12,191. The choice of procedure seems to be dictated at least in part by the size, nature, depth, and location of the lesions. Thus, to the extent possible it is important to obtain this information at the time of arthroscopy. The Maquet procedure123 is now rarely performed in Europe, its place of origin, perhaps because it had been used too enthusiastically and indiscriminately in patients with patellar pain and diffuse arthritis. This procedure preferentially unloads the distal half of the patella54 and may be more suited for distal patellar lesions. Its use has not been formally studied for trochlear lesions. Anteromedialization of the tibial tuberosity, popularized in the United States by Fulkerson62,64, seems best suited for distal-lateral lesions of the patella64. Lateral release and medial plication are indicated for lateral lesions78. Medial lesions are most troublesome, as malaligned patellae usually require medialization and this may increase the stresses on the arthritic areas. The Pridie technique of drilling the subchondral bone has yielded unpredictable results, as has the spongialization technique of Ficat et al.56, in which all subchondral bone is removed. Various types of partial patellectomy have been reported78,118, although without much corroboration or follow-up to date. These procedures seem to make eventual resurfacing more difficult. Patellectomy has been performed at least since 1860, sometimes on a relatively routine basis. This reflects an incomplete understanding of patellar mechanics by the surgeon24,41,91. Many variations of this procedure have been described12,32,171,185, and although some patients report a strong knee and an absence of pain, it is not a predictable procedure with regard to strength or pain relief. Moreover, it is a resection arthroplasty that cannot be readily reversed. Prosthetic resurfacing can provide excellent results when evaluated in the context of a knee replacement. Because of the lack of long-term results, there is a natural reluctance to perform isolated patellar resurfacing, especially in young patients.
Persistent pain following patellar realignment presents a unique challenge to the orthopaedic surgeon. In addition to considering all of the potential causes of patellar pain in a patient who has not been operated on, the surgeon must investigate whether the realignment was insufficient or excessive. Moreover, soft tissues may have stretched, rendering ineffective a realignment that was originally successful. Neuromas at the incision or portal sites132, patella baja143,152, and reflex sympathetic dystrophy must also be considered as possible causes of postoperative pain. Reflex sympathetic dystrophy may not be full-blown, as the classic features of hypoesthesia, coolness, and stiffness may be absent. Relief of pain with use of sympathetic blocks is diagnostic of reflex sympathetic dystrophy, and the blocks also may be curative78. Anesthetic injections in and about the knee can help to differentiate between intra-articular and extra-articular pathology.
Although patellar subluxation and dislocation are associated with well defined anatomical abnormalities, patellar pain is less clearly understood. It appears to be caused by a number of factors, which have a variable clinical expression. In adults, patellar malalignment is one of these factors. The nonoperative treatment of patellar malalignment must involve a program, specific to the patella, that is designed to stretch the hamstring muscles and the lateral knee structures, to bring the patella both actively and passively toward the center of the knee, to improve the strength and timing pattern of the quadriceps muscles, and to desensitize inflamed structures. In the few patients who require operative intervention, attempts should be made to address specific identifiable pathology. Note: The author especially wishes to thank the members of the Patellofemoral Study Group and guests, including P. Aichroth, A. Amis, E. Arendt, G. Bellier, H. Berieter, R. Biedert, D. Buuck, R. Cautilli, E. Cohen, J. Cox, D. Dejour, P. Djian, J.-Y. Dupont, Z. Duri, S. Dye, J. Farr, D. Fithian, N. Friederich, D. Fritschy, J. Fulkerson, C. Guier, J. Hart, R. Hoekman, D. Marshall, J. McConnell, A. Merchant, J. Minkoff, B. Moyen, T. Muellmer, E.-O. Muench, P. Neyret, W. Post, V. Sanchis-Alfonso, R. Scapinelli, H.-U. Stüubli, O. Siegrist, R. Teitge, G. Terry, N. Thomson, J. Valloton, and J. van Overschelde.
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Introduction
Etiology of Symptoms
,
and the aggregate modulus HA are related by the equation HA= E(1 - 
