This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and

Transcription

1 This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier s archiving and manuscript policies are encouraged to visit:

2 Journal of Biomechanics 42 (2009) Contents lists available at ScienceDirect Journal of Biomechanics journal homepage: The effect on patellofemoral joint stability of selective cutting of lateral retinacular and capsular structures Azhar M. Merican a,b, Eiji Kondo c,d, Andrew A. Amis a,c, a Musculoskeletal Surgery Department, Imperial College London, Charing Cross Hospital, London, UK b Department of Orthopaedic Surgery,University Malaya Medical Centre, Kuala Lumpur, Malaysia c Biomechanics Section, Mechanical Engineering Department, Imperial College London, London, UK d Department of Sports Medicine and Joint Reconstruction Surgery,Hokkaido University School of Medicine, Sapporo, Japan article info abstract Article history: Accepted 1 November 2008 Keywords: Patella Patellofemoral joint Stability Lateral release Retinaculum Patient selection for lateral retinacular release (LRR) and its efficacy are controversial. Iatrogenic medial subluxation can occur with inappropriate LRR. The aim of this study was to determine the reduction in patellofemoral stability with progressively more extensive LRR. The force required to displace the patella 10 mm medially and laterally in nine cadaveric knees was measured with and without loading of the quadriceps and iliotibial band. The knee was tested intact, then after progressive release beginning proximal to the patella (PR), the mid-level between the proximal and distal limit of the patella (MR) where the fibres are more transverse, then distally till Gerdy s tubercle (DR) and finally the joint capsule (CR). Both medial and lateral stability decreased with progressive releases, larger for the medial. The MR caused a significant reduction of lateral stability between 301 and 901 of knee flexion. There was an 8% reduction in medial stability at 01 flexion with a complete LRR (DR). A comparable reduction in medial stability in the loaded knee at 201 and 301 flexion was obtained with MR alone, with no further reduction after DR. A capsular release caused a further reduction in medial stability at 01 and 201 and this was marked in the unloaded knee. In extension, the main lateral restraint was the joint capsule. At 301 flexion, the transverse fibres were the main contributor to the lateral restraint. & 2008 Elsevier Ltd. All rights reserved. 1. Introduction The function of the lateral retinaculum remains incompletely understood. In contrast to the medial patellofemoral ligament, objective data on the biomechanical properties of the lateral retinaculum are lacking. Lateral retinacular release has been favoured because it is a relatively simple procedure. However, identifying the patient who will benefit from this surgery is not straightforward. Inappropriate lateral release can cause medial subluxation of the patella; this is a recognised complication (Brinker et al., 2001; Hughston and Deese, 1988; Hughston et al., 1996; Nonweiler and Delee, 1994). Limiting the lateral release to the required amount has been suggested to avoid this complication (Lattermann et al., 2007), and the use of staged lateral releases to balance the patella during total knee arthroplasty has been described (Strachan et al., 2008). Extensive release of the retinaculum including release of the vastus lateralis (longus) has been advocated (Paulos et al., 1980) but this muscle release was Corresponding author at: Department of Mechanical Engineering, Imperial College London, London SW7 2AZ, UK. Tel.: ; fax: address: a.amis@imperial.ac.uk (A.A. Amis). found in all cases of iatrogenic medial subluxation in one series (Nonweiler and Delee, 1994). Maintaining vastus lateralis obliquus (VLO) muscle support has been advised to reduce the complication of medial patellar subluxation (Fulkerson, 2002). It is preferable to minimise the extent of a surgical procedure while not compromising its effectiveness; at present, there is a lack of biomechanical data to guide the surgeon who may wish to reduce the extent of a lateral retinacular release. The clinician pushes the patella medially to gauge the tightness of the lateral retinaculum (Ford and Post, 1997) and the effectiveness of a release. A similar approach was used in a biomechanical comparison of lateral retinacular releases (Marumoto et al., 1995) in which increased medial translation of the patella with constant force was found when the release was extended distally to the tibial tubercle. Patellar stability can be quantified by measuring the force that opposes the linear displacement of the patella from its initial position of equilibrium (Farahmand et al., 1998b; Senavongse et al., 2003). This provides a more direct method to quantify patellar stability than inferring it indirectly from changes of joint contact pattern or patellar tracking. Patellofemoral joint stability depends on factors that interact in a complex manner, including the extensor mechanism, the retinacular restraints, the articular geometry and the limb alignment. Patellar medial stability should decrease with a lateral /$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi: /j.jbiomech

3 292 A.M. Merican et al. / Journal of Biomechanics 42 (2009) release; this is a reflection of its effectiveness. However, lateral release also decreases lateral stability (Christoforakis et al., 2006; Desio et al., 1998), so it may not be suitable for the treatment of lateral patellar maltracking or instability (Christoforakis et al., 2006). These studies, however, failed to investigate its contribution to medial stability. The clinical evidence (Brinker et al., 2001; Hughston and Deese, 1988; Hughston et al., 1996; Nonweiler and Delee, 1994) shows that it is important not to reduce medial stability excessively. To characterise the role of lateral retinacular release in patellar stability fully, both medial and lateral stability need to be examined. Patellar lateral instability is a common clinical problem, while medial instability is unusual and has been linked with inappropriate lateral retinacular releases. Thus, it is important that the surgeon should not inadvertently trigger either medial or lateral patellar instability, this study was designed to quantify the contribution of the various parts of the lateral retinaculum to patellar stability and to test the hypothesis that the dominant component is the transversely oriented ITB-P fibres of the mid-part of the lateral retinaculum. 2. Methods Nine fresh-frozen cadaveric knees with no history of knee surgery or disease were used (mean age years; range 41 85). These were obtained from the International Institute for the Advancement of Medicine (Jessup, PA, USA). The Institute undertook screening and consent for use in research. The study was approved by the Riverside Research Ethics Committee. The knees were stored at 20 1C and thawed a day prior to experimentation. The skin and subcutaneous tissue were removed. The deep fascia, retinaculae and ITB were preserved. The femur and tibia were cut approximately 20 and 15 cm above and below the knee, respectively. The head of the fibula was transfixed to the tibia by two screws to maintain its anatomical position and then the distal part excised. An intramedullary sleeve and a rod were cemented into the femur and tibia, respectively. The sleeve in the femur was aligned to the central axis by use of an outrigger alignment rod. A spherical polyethylene socket was cemented into the patella centred over the median ridge and 10 mm deep to the anterior (superficial) surface. This was taken to be the geometric centre of the patella. The stability rig was composed of two parts. The fixed part was attached to the base of an Instron materials testing machine (Instron Ltd., High Wycombe, UK) and the moving part was a three degree-of-freedom mounting attached to the Instron load cell (Fig. 1). The knee was mounted sideways (lateral aspect upwards) in the fixed part of the stability rig by locating the cemented femoral sleeve onto a rod. The knee was aligned with the anatomical axis of the femur perpendicular to the load cell axis. Femoral rotation was fixed with the most-posterior parts of the femoral condyles vertical in a distal proximal view. The lower end of the three degree-of-freedom mounting was snap-fitted into the polyethylene patellar socket to form a ball and socket joint (Fig. 1). The mounting allowed for patellar mobility in a sagittal plane: anterior posterior and proximal distal translations, and flexion extension rotation and the ball and socket joint allowed unimpeded tilt and rotations when the patella was displaced medially and laterally. The components of the quadriceps were separated and loaded with hanging weights using cables and pulleys. A total load of 175 N was applied to the muscle groups (Farahmand et al., 1998b). This was done according to the directions and physiological cross-sectional areas (PCSAs) of the muscles (Farahmand et al., 1998a): VLL 141 lateral and 01 anterior; VLO 351 lateral and 331 posterior; VML 151 medial and 01 anterior; VMO 471 medial and 441 posterior; and RF+VI 01 lateral and 01 anterior. The tension distribution was: RF+VI 35%, VLL 33%, VLO 9%, VML 14%, and VMO 9%. Thirty Newtons tension was applied to the ITB, directed 01 lateral and 61 posterior to the femoral axis (Bull et al., 1999). For the passive knee, nominal tensions were used to keep the muscle taut and to minimise the effect of gravity on the patella. The muscle was loaded in the same directions and proportions with 30 N in total and the ITB with 5 N. The knee was tested at 01, 201, 301, 601, and 901 flexion. It was flexed against the muscle tension and at the chosen test angle a vertical rod was placed anterior to the tibial rod to block knee extension. The interface was greased allowing the tibia to rotate axially and translate during testing. The patella was displaced cyclically 10 mm laterally and medially at 100 mm/min from its stable neutral position. This displacement was repeated four times in each test and the fourth load vs. displacement curve was recorded. The data analysed were the forces measured by the load cell at the medial and lateral limits of the displacement cycle. These forces defined the patellar medial and lateral stability, respectively, for that particular knee flexion angle. Testing was repeated after each stage of lateral release. The anatomy of the lateral retinaculum has been re-examined by the authors (Merican and Amis, 2008) and the stages of the lateral release were based on this Proximal release (PR) The lateral extension of the quadriceps aponeurosis anchored the deep fascia close to where it thickened to form the iliotibial band and thus was the proximal beginning of the interaction of the quadriceps aponeurosis and the iliotibial band. This was incised releasing the connection between lateral retinaculum and quadriceps tendon proximal to the level of the proximal pole of the patella Middle release (MR) The lateral retinaculum was incised lateral to the patella from the level of the proximal pole to the distal pole of the patella. The VLO tendon was not cut but the connection between ITB and the distal edge of this tendon was disrupted. This release cut the thickest part of the retinaculum, which was reinforced by the deeper transverse fibres that connected the ITB to the lateral patella (ITB-P fibres) and the VLO Distal release (DR) The distal part of the retinaculum, predominantly longitudinal in orientation, was cut from the level of the distal pole of the patella to Gerdy s tubercle. There was now no connection between ITB and the patella or quadriceps mechanism (Fig. 2) Capsular release (CR) The lateral capsule was incised. Cutting the thickenings of the capsule; the lateral patellofemoral and patellomeniscal ligaments. The stability of the patella was examined at each flexion angle and stage of the lateral retinacular release using a two-way repeated-measures analysis of variance. Bonferroni post-hoc tests were used to determine the knee flexion angles and stages of the lateral release which caused significant changes in patellar stability. Significance level was set at po Results Fig. 1. A diagrammatic representation of the experimental setup. The knee is secured to a muscle loading rig attached to an Instron materials testing machine. The Instron load cell moves a three degree-of-freedom mounting. Its lower end is connected to the patella via a snap-fit ball and socket joint (inset). The socket is made of polyethylene and is cemented into the patella; its centre corresponds to the geometric centre of the patella. Generally, there was a decrease in medial and lateral stability with progressively more extensive release. The decrease in medial stability was larger than in lateral stability. After proximal release, the reductions in medial and lateral stability were not statistically significant in the loaded or passive knee across the range of flexion.

4 A.M. Merican et al. / Journal of Biomechanics 42 (2009) Fig. 2. The photo on the left shows an experimental knee in the rig and Instron materials testing machine after a complete release of the lateral retinaculum. The edges of the retinaculum (dark arrows) have retracted exposing the underlying intact joint capsule. The diagram on the right shows the levels of release of the lateral retinaculum. The capsular release follows (not shown) by release of the deeper lateral capsule layer. Medial stability in the intact loaded knee increased progressively with knee flexion, from a mean of 78 N at 01 to 171 N at 901 flexion. After the proximal release, a statistically significant effect on medial stability was not seen. When the release was extended distally to the level of the distal pole of the patella (MR), there was a significant reduction in medial stability, at 201, 301, and 901 flexion. There was a reduction of 6 N74 at201 and 301 flexion and 11 N713 at 901 flexion compared to the intact knee. This corresponds to a 7% decrease in medial stability compared to the intact knee at 201, 301, and 901 flexion (Fig. 3b). A significant reduction compared to the intact knee was seen at 01 when the release was extended distally (DR); 6 N74 or 8%. There was no change in medial stability at 201 and 301 flexion when the lateral retinacular release was extended distally from a middle release to a distal release (Fig. 3a). The addition of a capsular release caused significant change in the medial stability at 01, 201, and 901 compared to the stability of the knee with a middle release (Fig. 3a). At 01 and 201 flexion, on average, after a capsular release there was a reduction of medial stability of 13 N79 or 16% compared to the intact knee (Fig. 3b). At , there was no significant change in medial stability with a capsular release as compared to a middle (MR) or complete retinacular release without the capsule (DR) (Fig. 3a). With the knee extended, the main lateral retinacular restraint to patellar medial translation was the lateral joint capsule. However, if the whole lateral retinaculum was released, a comparable reduction in medial stability was also achieved. At 201 flexion, the deeper transverse fibres (ITB-P) and the capsule had comparable contributions to restraining patellar medial translation and by 301 flexion, the transverse fibres were the main contributor (Fig. 3b). In the passive condition, the effect of capsular release was exaggerated (Fig. 4). The mean lateral stability for the intact loaded knee ranged from 76 to 100 N across flexion, being lowest at 201 The trend was similar with the passive knee, when the mean stability ranged from 33 to 55 N. For the loaded knee, there was a statistically significant reduction in the lateral stability (compared to the intact knee) between 301 and 901 flexion for the middle release, 201 and 901 for the distal release and 01 and 901 for the complete release including the capsule (CR). After a middle release, the lateral stability reduced by an average of 5%75 of the Fig. 3. (a) Reduction in medial stability in the loaded knee after consecutively more release of the lateral restraint expressed as a percentage of the medial stability of the intact knee. Bonferroni post-hoc test compared the change in restraining force to the immediately preeceding status (i.e., proximal release vs. intact, middle release vs. PR, distal release vs. MR, capsular release vs. DR) * ¼ po0.05, ** ¼ po0.001, *** ¼ po (b) The change in patellar medial stability after consecutive lateral releases as a contribution to patellar medial stability (expressed as a percentage of the intact knee stability). stability of the intact knee across (Fig. 5b). When the retinaculum was completely released (DR) there was no significant change compared to the middle release (Fig. 5a). However, the change in stability compared to the intact knee was now statistically significant at 201 too. The decrease of patellar lateral stability after releasing the retinaculum but not the joint capsule (DR) was on average 6%76 of the intact knee stability. Releasing the capsule (CR) reduced the lateral stability further, but the change was not significant compared to the stability after the middle release (Fig. 5a). On average, the reduction in lateral stability across was 7%74% compared to the intact knee for the retinacular plus capsular release. For the passive knee, a middle release caused a significant reduction in lateral stability compared to the intact knee at 01 as well as from 301 to 901 On average across the range of flexion, the reduction in lateral stability compared to the intact knee was

5 294 A.M. Merican et al. / Journal of Biomechanics 42 (2009) Fig. 4. (a) Reduction in medial stability in the passive knee after consecutively more release of the lateral restraint expressed as a percentage of the medial stability of the intact knee. Bonferroni post-hoc test compared the change in restraining force to the immediately preeceding status (i.e., proximal release vs. intact, middle release vs. PR, distal release vs. MR, capsular release vs. DR) * ¼ po0.05, ** ¼ po0.001, *** ¼ po (b) The change in patellar medial stability after consecutive lateral releases as a contribution to patellar medial stability (expressed as a percentage of the intact knee stability). 7%77% after the middle release and 12%76% for the complete release including the capsule (CR) (Fig. 6). 4. Discussion This study found that staged release of the lateral retinacula reduced the medial stability of the patellofemoral joint progressively, making it easier to push the patella medially. The finding that the mid-part of the retinaculum, lateral to the patella, contributed significantly to the medial stability of the patella is in keeping with the anatomy and our hypothesis. In this region, the retinaculum is dense and thicker, with fibres that are predominantly transverse in orientation (Merican and Amis, 2008). These fibres anchor the lateral patella and the VLO tendon to the ITB, Fig. 5. (a) Reduction in lateral stability in the loaded knee after consecutively more release of the lateral restraint expressed as a percentage of the lateral stability of the intact knee. Bonferroni post-hoc test compared the change in restraining force to the immediately preeceding status (i.e., proximal release vs. intact, middle release vs. PR, distal release vs. MR, capsular release vs. DR) * ¼ po0.05. (b) The change in patellar lateral stability after consecutive lateral releases expressed as a contribution to patellar lateral stability (expressed as a percentage of the intact knee stability). resisting medial translation of the patella. The retinaculum distally is less thick and predominantly longitudinal in orientation, which may explain its smaller contribution to medial lateral stability. The lateral capsule of the knee is relatively thin but is reinforced by thickenings, although these are variable and not universally present (Blauth and Tillmann, 1983; Fulkerson and Gossling, 1980; Merican and Amis, 2008). It was, therefore, not expected that the capsule would contribute so much to lateral restraint, particularly in extension. In-vitro work has not shown a change in patellofemoral contact area or pressure with lateral retinacular release (Hille et al., 1985; Huberti and Hayes, 1988; Lewallen et al., 1990). Ostermeier et al. (2007) found that with lateral release, the patella was lateralised in the flexion range and medialised beyond 601. There was no significant change in contact pressure. These measurements, however, are only indirect indicators of patellar stability. Patellar stability can be described quantitatively as the

6 A.M. Merican et al. / Journal of Biomechanics 42 (2009) study. This study was also limited by only examining patellar lateral displacement. The discussion emphasised that, although lateral retinacular release may have beneficial effects in patellar compression syndrome, it did not appear to have a role in addressing lateral instability. This study has inherent limitations. As with most other studies in-vitro, it used elderly knees and it is not known how well their behaviour extrapolates to the younger patients for whom lateral release procedures are considered. Although care was taken to tense the heads of the quadriceps in a physiological manner, their relative contributions vary from knee to knee and the overall tension was limited by tearing the muscle fibres in these elderly specimens. We also do not know the physiological loading appropriate for the ITB, but similar proportions of ITB tension to quadriceps loading have been used in other works (Bull et al., 1999; Kwak et al., 2000; Yamamoto et al., 2006). These knees were normal for age: in diseased knees pathological bands or other changes may alter the relative contributions of restraint of the capsule and lateral retinaculum. In these normal experimental knees, the largest reduction in medial stability in the loaded knee at 301 flexion was obtained with the release of the deep transverse fibres that linked the iliotibial tract to the patella and VLO, with no further increase with subsequent releases. It is difficult to say at what point the reduction of medial stability attained from release is too effective and there is a danger of medial subluxation. The marked reduction in medial stability with a retinacular and capsular release in the passive extended knee (Fig. 4) reflects the absence of bony constraint on the medial side in extension. This helps to explain why in the cases of misdiagnosis, an extensive lateral release can lead to medial subluxation. Conflict of interest None Acknowledgements Fig. 6. (a) Reduction in lateral stability in the passive knee after consecutively more release of the lateral restraint expressed as a percentage of the lateral stability of the intact knee. (b) The change in patellar lateral stability after consecutive lateral releases as a contribution to patellar lateral stability (expressed as a percentage of the intact knee stability). Azhar M. Merican was supported by the University of Malaya Medical Centre, Kuala Lumpur and the Arthritis Research Campaign (ARC). The ARC donated the Instron machine and the cost of materials and knee specimens was funded by a grant from the ARC. We also thank Phillip Wilson for his technical support. displacement induced by an applied load (Fithian et al., 1995; Marumoto et al., 1995; Skalley et al., 1993) or, as in this study, the force needed to displace the patella by a fixed amount. Marumoto et al. (1995) found that lateral retinacular release was more effective when it extended distally to the level of the tibial tubercle, allowing significantly more medial displacement of the patella. That finding differs from this study, possibly because the quadriceps was only loaded to 10 N parallel to the femur. Furthermore, the ITB was not loaded in that experiment and its loading affects patellar kinematics (Kwak et al., 2000). The only other quantitative study of the effect of lateral releases on patellar stability (Christoforakis et al., 2006) found a larger reduction of lateral stability than in this study: a complete lateral retinacular release reduced patellar lateral stability by 16 19% across knee flexion. The finding that the significant effect was near knee extension was in line with clinical experience of patellar instability. The only mechanical factor that might explain this difference is that there was no loading of the ITB in the earlier References Blauth, M., Tillmann, B., Stressing on the human femoro patellar joint. I. Components of a vertical and horizontal tensile bracing system. Anat. Embryol. (Berlin) 168 (1), Brinker, M.R., O Connor, D.P., Flandry, F., Hughston, J.C., Diagnosis and surgical correction of medial patellar subluxation. Oper. Tech. Sports Med. 9 (3), Bull, A.M., Andersen, H.N., Basso, O., Targett, J., Amis, A.A., Incidence and mechanism of the pivot shift. An in vitro study. Clin. Orthop. Relat. Res. (363), Christoforakis, J., Bull, A.M., Strachan, R.K., Shymkiw, R., Senavongse, W., Amis, A.A., Effects of lateral retinacular release on the lateral stability of the patella. Knee Surg. Sports Traumatol. Arthroscopy 14 (3), Desio, S.M., Burks, R.T., Bachus, K.N., Soft tissue restraints to lateral patellar translation in the human knee. Am. J. Sports Med. 26 (1), Farahmand, F., Senavongse, W., Amis, A.A., 1998a. Quantitative study of the quadriceps muscles and trochlear groove geometry related to instability of the patellofemoral joint. J. Orthop. Res. 16 (1), Farahmand, F., Tahmasbi, M.N., Amis, A.A., 1998b. Lateral force displacement behaviour of the human patella and its variation with knee flexion a biomechanical study in vitro. J. Biomech. 31 (12), Fithian, D.C., Mishra, D.K., Balen, P.F., Stone, M.L., Daniel, D.M., Instrumented measurement of patellar mobility. Am. J. Sports Med. 23 (5),

7 296 A.M. Merican et al. / Journal of Biomechanics 42 (2009) Ford, D.H., Post, W.R., Open or arthroscopic lateral release. Indications, techniques, and rehabilitation. Clin. Sports Med. 16 (1), Fulkerson, J.P., Diagnosis and treatment of patients with patellofemoral pain. Am. J. Sports Med. 30 (3), Fulkerson, J.P., Gossling, H.R., Anatomy of the knee joint lateral retinaculum. Clin. Orthop. Relat. Res. (153), Hille, E., Schulitz, K.P., Henrichs, C., Schneider, T., Pressure and contractsurface measurements within the femoropatellar joint and their variations following lateral release. Arch. Orthop. Trauma Surg. 104 (5), Huberti, H.H., Hayes, W.C., Contact pressures in chondromalacia patellae and the effects of capsular reconstructive procedures. J. Orthop. Res 6 (4), Hughston, J.C., Deese, M., Medial subluxation of the Patella as a complication of lateral retinacular release. Am. J. Sports Med. 16 (4), Hughston, J.C., Flandry, F., Brinker, M.R., Terry, G.C., Mills, J.C., Surgical correction of medial subluxation of the patella. Am. J. Sports Med. 24 (4), Kwak, S.D., Ahmad, C.S., Gardner, T.R., Grelsamer, R.P., Henry, J.H., Blankevoort, L., Ateshian, G.A., Mow, V.C., Hamstrings and iliotibial band forces affect knee kinematics and contact pattern. J. Orthop. Res. 18 (1), Lattermann, C., Toth, J., Bach Jr., B.R., The role of lateral retinacular release in the treatment of patellar instability. Sports Med. Arthroscopy 15 (2), Lewallen, D.G., Riegger, C.L., Myers, E.R., Hayes, W.C., Effects of retinacular release and tibial tubercle elevation in patellofemoral degenerative joint disease. J. Orthop. Res. 8 (6), Marumoto, J.M., Jordan, C., Akins, R., A biomechanical comparison of lateral retinacular releases. Am. J. Sports Med. 23 (2), Merican, A.M., Amis, A.A., Anatomy of the lateral retinaculum of the knee. J. Bone Jt. Surg. Br. Vol. 90B (4), Nonweiler, D.E., Delee, J.C., The diagnosis and treatment of medial subluxation of the Patella after lateral retinacular release. Am. J. Sports Med. 22 (5), Ostermeier, S., Holst, M., Hurschler, C., Windhagen, H., Stukenborg-Colsman, C., Dynamic measurement of patellofemoral kinematics and contact pressure after lateral retinacular release: an in vitro study. Knee Surg. Sports Traumatol. Arthroscopy 15 (5), Paulos, L., Rusche, K., Johnson, C., Noyes, F.R., Patellar malalignment: a treatment rationale. Phys. Ther. 60 (12), Senavongse, W., Farahmand, F., Jones, J., Andersen, H., Bull, A.M., Amis, A.A., Quantitative measurement of patellofemoral joint stability: force displacement behavior of the human patella in vitro. J. Orthop. Res. 21 (5), Skalley, T.C., Terry, G.C., Teitge, R.A., The quantitative measurement of normal passive medial and lateral patellar motion limits. Am. J. Sports Med. 21 (5), Strachan, R.K., Merican, A.M., Devadasan, B., Maheshwari, R., Amis, A.A., A technique of staged lateral release to correct patellar tracking in total knee arthroplasty. J. Arthroplasty 2008 May 19. [Epub ahead of print]. Yamamoto, Y., Hsu, W.H., Fisk, J.A., Van Scyoc, A.H., Miura, K., Woo, S.L., Effect of the iliotibial band on knee biomechanics during a simulated pivot shift test. J. Orthop. Res. 24 (5),