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Letters to the Editor to:

Scientific Articles:
Samuel R. Ward, Michael R. Terk, and Christopher M. Powers
Patella Alta: Association with Patellofemoral Alignment and Changes in Contact Area During Weight-Bearing
J Bone Joint Surg Am 2007; 89: 1749-1755 [Abstract] [Full text] [PDF]
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[Read Letter to the Editor] Dr. Ward and Dr. Powers respond to Dr. Rogers
Samuel R. Ward, PhD, PT, Christopher M. Powers, PhD, PT   (20 September 2007)
[Read Letter to the Editor] Patella Alta: Association with Patellofemoral Alignment and Changes in Contact Area
Benedict A. Rogers, MA, MSc, MRCS   (16 August 2007)

Dr. Ward and Dr. Powers respond to Dr. Rogers 20 September 2007
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Samuel R. Ward, PhD, PT,
Assistant Professor of Radiology and Orthopaedic Surgery
University of California San Diego,
Christopher M. Powers, PhD, PT

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Re: Dr. Ward and Dr. Powers respond to Dr. Rogers

srward{at}ucsd.edu Samuel R. Ward, PhD, PT, et al.

To The Editor:

We would like to thank Benedict A. Rogers, MA, MSc, MRCS, for his insightful questions about our recent manuscript. Each point brought forth in the original letter is addressed below.

1) There are two papers by Moller et al.(1,2) referenced in this manuscript. As Mr. Rogers correctly points out, one demonstrates the relationship between patellofemoral malalignment and histological signs of articular cartilage damage in a rabbit model(1), which is reference “6” in the manuscript. The second paper, demonstrates the relationship between patellar height and patellofemoral incongruence in human knees (2), which is references “23” in the manuscript. Reference “6” was incorrectly cited in the introduction (line 18). We apologize for this oversight, however, a thorough discussion of our findings relative to Dr. Moller’s human data (correctly cited) is included in the Discussion section.

2) One investigator measured the Insall-Salvati index in all cases. However, this investigator was blinded to the alignment and contact area measurements. This index has demonstrated interobserver reliability in our hands and has been shown to be comparable to measurements made on lateral radiographs of the knee(3).

3) The Insall-Salvati index(4) was originally measured on lateral radiographs of the knees flexed to 20-30 degrees as Mr. Rogers correctly notes. The purpose of flexing the knee was to remove slack from the extensor mechanism and allow the patella to engage with the trochlea. In our experiment, the leg was loaded in 0 degrees of knee extension with 25% of the subject’s body weight(5), which removes slack from the extensor mechanism. Regarding engagement with the femoral trochlea, we measured the height of the patella with the knee in extension and at 20 degrees of flexion in each subject and found that these measurements to be nearly identical (ICC = 0.92).

4) The Insall-Salvati index(4) does lack sensitivity to patellar morphology as Dr. Rogers correctly notes. Interestingly, our original hypotheses about the lack of correlation between the height of the patella and the amount of patellofemoral malalignment included suboptimal measurements of patellar height. For this reason we originally measured patellar height using a variety of published indices, including those noted in Mr. Roger’s JBJS manuscript. However, none of them had associative values with malalignment and contact area that were as strong as the Insall-Salvati index. This was in direct contradiction to our original hypothesis. In response, we went back to our original data and determined that large Insall-Salvati indices were always driven by long patellar ligament lengths and not by patellar geometry(6). Although we acknowledge this has been reported in the literature, it was not apparent in our data. Therefore, the most simple measurement (Insall-Salvati) explained the largest amount of the variance in alignment and contact area. Perhaps this is why the measurement has survived in practice for over 35 years.

References:

1. Moller, B. N.; Moller-Larsen, F.; and Frich, L. H.: Chondromalacia induced by patellar subluxation in the rabbit. Acta Orthop Scand, 60(2): 188-191, 1989.

2. Moller, B. N.; Krebs, B.; and Jurik, A. G.: Patellar height and patellofemoral congruence. Arch Orthop Trauma Surg, 104(6): 380-381, 1986.

3. Miller, T. T.; Staron, R. B.; and Feldman, F.: Patellar height on sagittal MR imaging of the knee. AJR Am J Roentgenol, 167(2): 339-341, 1996.

4. Insall, J., and Salvati, E.: Patellar position in the normal knee joint. Radiology, 101: 101-104, 1971.

5. Ward Sr, Terk Mr, Powers CM. Patella alta: Association with patellofemoral alignment and changes in contact area during weight-bearing. J Bone Joint Surg Am. 2007;89:1751.

6. Ward Sr, Terk Mr, Powers CM. Patella alta: Association with patellofemoral alignment and changes in contact area during weight-bearing. J Bone Joint Surg Am. 2007;89:Table 1.
Patella Alta: Association with Patellofemoral Alignment and Changes in Contact Area 16 August 2007
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Benedict A. Rogers, MA, MSc, MRCS,
Specialist Registrar
The Princess Royal Hospital, Haywards Heath, UK

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Re: Patella Alta: Association with Patellofemoral Alignment and Changes in Contact Area

benedictrogers{at}hotmail.com Benedict A. Rogers, MA, MSc, MRCS

To The Editor:

I read with interest the paper by Ward et al.(1) entitled, “Patella Alta: Association with Patellofemoral Alignment and Changes in Contact Area During Weight Bearing”.

I would like to make the following points:

1. In the introduction, the paper references a study by Moller et al.(2) conducted on rabbits purporting a weak association between the position of the patella relative to the femur and patellofemoral malalignment. However, this statement may not be applicable to the human patella since substantial anatomical and biomechanical differences exist between the human and rabbit knee. In addition, the paper by Moller et al.(2) studied the histological changes induced by subluxation of the patella rather than the biomechanical associations between patellar alta and patellofemoral malalignment.

2. The study (1) utilizes the Insall-Salvati ratio to diagnosis patella alta from a sagittal radiograph of the knee. However, the study fails to detail how many observers were used for this. The Insall-Salvati ratio has been shown to have a significantly higher interobserver error compared to both the Blackburne-Peel and Caton – Deschamps ratios(3).

3. Insall’s original study(4) measured patella height with the knee in 20 to 30 degrees of flexion to ensure tension existed within the extensor mechanism and to allow for the patella to engage within the femoral trochlea. How can this study assess patella height using the Insall- Salvati ratio with the knee in 0 degress flexion?

4. Further, the Insall-Salvati ratio lacks sensitivity to patellar morphology leading to inaccurate readings for patellar height in patients whose patella exhibits a long distal (nonarticulating) facet. As a result this ratio has been modified by Grelsamer(5) to account for these anatomical variations. Was any assessment made of patellar morphology in this study to ensure that the Insall-Salvati ratio used provided an accurate measure of patellar height?

The author did not receive any outside funding or grants in support of his research for or preparation of this work. Neither he nor a member of his immediate family received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the author, or a member of his immediate family, is affiliated or associated .

References:

1. Ward SR, Terk MR, Powers CM. Patella alta: association with patellofemoral alignment and changes in contact area during weight- bearing. J Bone Joint Surg Am 2007;89:1749-1755.

2. Moller BN, Moller-Larsen F, Frich LH. Chondromalacia induced by patellar subluxation in the rabbit. Acta Orthop Scand 1989;60:188-191.

3. Rogers BA, Thornton-Bott P, Cannon SR et al. Interobserver variation in the measurement of patellar height after total knee arthroplasty. J Bone Joint Surg Br 2006;88:484-488.

4. Insall J, Salvati E. Patella position in the normal knee joint. Radiology 1971;101:101-104.

5. Grelsamer RP, Meadows S. The modified Insall-Salvati ratio for assessment of patellar height. Clin Orthop 1992;170-176.