Author's Accepted Manuscript Biomechanics of the Patellofemoral Joint Jack Andrish MD PII: DOI: Reference: www.elsevier.com/locate/enganabound S1060-1872(15)00021-0 http://dx.doi.org/10.1053/j.otsm.2015.03.001 YOTSM50478 To appear in: Oper Tech Sports Med Cite this article as: Jack Andrish MD, Biomechanics of the Patellofemoral Joint, Oper Tech Sports Med , http://dx.doi.org/10.1053/j.otsm.2015.03.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Biomechanics of the Patellofemoral Joint Jack Andrish, MD Department of Orthopaedic Surgery Cleveland Clinic ABSTRACT: The biomechanics and kinematics of the patellofemoral joint are the result of a complex assortment of static and dynamic conditions. This chapter will present a review of our basic understanding of these conditions and place them in the context of necessary information required when considering surgical interventions. The importance of this information becomes highlighted when one considers that the knee is a coupled mechanical system and changes in any one component of the system can affect any of the remaining parts of the system. INTRODUCTION: The biomechanical characteristics of the patellofemoral joint are the result of a complex interplay of components. The statics and dynamics of this articulation involve the geometries of the patella and the trochlea, the passive soft tissue restraints of capsule and retinaculum, and the coordination of the quadriceps. Additionally, angular and rotational limb alignment can affect patellofemoral mechanics and kinematics. Bottom line - it’s all about balance; balance of the extensor mechanism of the knee. As we proceed through this paper, we should take note of some advice I received many years ago from an engineering and research colleague. “The knee is a coupled mechanical system. One cannot change any one part without affecting the remaining parts of the system.” (Tony Valdevit; personal communication) Many of the subsequent studies referenced within this chapter will support the advice I had received. Finally, the biomechanical descriptions to follow will be generally qualitative interpretations and I would refer to the appropriate studies referenced for more quantitative data. RELEVANT OSSEOUS ANATOMY: The patella has been considered to be a large sesamoid bone lying within the extensor mechanism of the knee. The posterior surface consists of two major concave facets, medial and lateral, with a small convex medial “odd facet.” The lateral facet is typically larger, although Wiberg has described a classification of patellar shape that generally fits most presentations.1 (Figure 1.) Type 1 describes a patella with equal size of medial and lateral facets. Type 2 has the lateral facet larger than the medial, and Type 3 depicts a large lateral facet with a small, more vertically oriented medial facet. And then as we will subsequently see, there is a very small vertically oriented “odd facet” that is adjacent to the medial facet which makes contact only in deep flexion. Panni, et al, have described the increased prevalence of the Type 3 patella in patients with recurrent dislocations of the patella and the association with certain types of trochlear dysplasia.2 The thickest articular cartilage in the body is found within the patellofemoral joint. On the patella, it can measure 4 to 6 millimeters in depth and is progressively thicker proximally.3-5 The trochlear geometry has been studied extensively.6-11 Amis and his colleagues have determined that the primary restraint to lateral translation of the patella, once engaged within the trochlea and beyond 30 degrees of flexion, is the slope of the lateral wall of the trochlea.11 Dejour et al9 have developed a working classification of trochlear shape. Type A is characterized by a shallow trochlea and a positive “crossing sign” seen on the lateral radiograph. Type B has a shallow/flat trochlea as well as a supra trochlear spur. Type C has a positive crossing sign and a double contour sign seen on the lateral radiograph, a convex lateral trochlear facet and hypoplasia of the medial trochlear facet. Type D has everything that Type C has, plus a supra trochlear spur and marked hypoplasia of the medial trochlear facet. RELEVANT SOFT TISSUE ANATOMY: The extensor mechanism of the knee is composed of the quadriceps muscles and their respective tendons, the patellar tendon, and the medial and lateral retinaculum. The vastus medialis extends further distally on the patella than the tendon of the vastus lateralis. The vastus medialis oblique (VMO) insertion has a more horizontal rather than vertical orientation. A small component attaching to the proximal/lateral boarder of the patella can be referred to as the vastus lateralis oblique and has muscular origins from the lateral intermuscular septum. Farahmand et al11 have described the relative influences of the components of the quadriceps muscles. They estimated force contributions based upon cross-sectional areas. On that basis, they felt that the rectus femoris and vastus intermedius contribute 35% of total quadriceps strength; the vastus medialis contributes 25% and the vastus lateralis (although the most variable in cross-sectional area) 40%. The length of the patellar tendon can be variable. A number of methods of measuring patellar height have been validated.12 Ranges of normal have been established. Indices that depict the patellar tendon as being too long are referred to as patella alta; while indices that demonstrate the patellar tendon as being too short are referred to as patella baja or patella infera. The effects upon patellofemoral loading will be discussed later. Fulkerson has described the anatomy of the lateral retinaculum and demonstrated a thin, oblique component with a stout deep transverse component.13 A lateral epicondylopatellar ligament has been described with variable prevalence. The deep transverse lateral retinaculum takes its origin from the deep surface of the iliotibial band. Warren and Marshall described three fascial layers of the medial aspect of the knee.14 The most superficial layer encloses the vastus medialis and extends distally to be the sartorius fascia. The intermediate layer includes the medial patellofemoral ligament and distally includes the superficial medial collateral ligament. The deep layer includes the joint capsule. Considerable attention recently has been given to a whisp of a structure within the intermediate layer.15-20 This has been defined as the medial patellofemoral ligament (MPFL) with a variable length of attachment to the proximal half of the medial boarder of the patella and a femoral origin within a saddle between the adductor tubercle and the medial femoral epicondyle. Baldwin has confirmed the transverse component of the MPFL originating between the adductor tubercle and the medial femoral epicondyle, but also an oblique extension that blends with the anterior boarder of the superficial medial collateral ligament.15 Proximally, the anterior half to two thirds of the transverse band of the MPFL blends with the posterior surface of the VMO. Whether it is the medial or lateral retinacular structures, it is important to understand their mechanical effects by viewing their orientations. We too often view their orientations in the coronal plane, but when viewed in the axial plane, it becomes more understandable how they both function to assist engagement of the patella within the trochlea. (Figure 2.) MECHANICS: Ostensibly, the patella acts to enhance the pulley effect by increasing the moment arm distance from the extensor mechanism to the instant center of motion of the knee. Indeed, this effect can improve the efficiency of the quadriceps by as much as 50 percent.3,21 The patella also serves to centralize the divergent forces of the quadriceps during flexion as the patella engages within the trochlea. With the knee in extension, the angle formed by the resultant quadriceps force and the patellar tendon is known as the Qangle. This provides a force to laterally displace the patella, while at the same time counters external rotation of the tibia.7,22 During knee flexion, the obligatory internal rotation of the tibia reduces the Q-angle and thus reduces the forces that produce lateral displacement. In this sense, the patella is most vulnerable to dislocation during the initial 30 degrees of knee flexion when it is less secure within the trochlea and when the Qangle is greatest. Medializing the tibial tuberosity results in reducing the Q-angle, but studies have shown that it also results in an increase in external tibial rotation.22,23 Huberti et al24 have assessed the force distributions within the quadriceps tendon and the patellar tendon during flexion and extension. (Figure 3.) They concluded that the patella does not act like a simple pulley, but the distributions of forces are dependent upon the changing size and locations of patellofemoral contact during motion. They analyzed the ratio of patellar tendon force to quadriceps force and found that as the knee comes into extension, the force within the patellar tendon increases and is greater than the force within the quadriceps tendon. The opposite occurs with knee flexion where the force within the quadriceps tendon is greater than the patellar tendon. The crossover point for this force distribution occurs around 45 degrees of knee flexion. Considerable effort has been placed into understanding the forces and pressures sustained by the patellofemoral joint.3-5,8,10,25-29 Estimates are that with activities of daily living, the patella encounters forces ranging from 0.5 to 9.7 times body weight; and that with some sports activities the forces can approach 20 times body weight.3 Figure 4 depicts the vector diagram used to estimate patellofemoral reaction force. The patellofemoral reaction (contact) force is equal to and opposite to the resultant of the quadriceps and patellar tendon forces. It is also important to visualize the influence of body position and knee flexion angle on patellofemoral reaction force3 as depicted in Figure 4. As weight bearing knee flexion increases, the ground reaction force through the center of gravity of the body produces an increase in the moment arm of the femur and tibia resulting in increasing patellofemoral reaction force. Leaning forward, however, brings the center of gravity forward, reducing the moment arm and thus reducing patellofemoral reaction force. The patella also serves to increase the area of force distribution. The area of patellar contact is not uniform throughout the range of motion. (Figure 5.) When the knee is fully extended the patella is just proximal and slightly lateral to the trochlea. As the knee flexes and the patella engages within the trochlea, the load bearing begins at the most distal patellar surface and progresses proximally until it reaches the maximum amount of contact area at 80 to 90 degrees of flexion.5 Therefore, as patellofemoral reaction force increases, so does the contact area, resulting in modulating stress (load per unit area). In the mid-range of knee flexion, between 50 to 90 degrees of flexion, the quadriceps tendon begins to turn around the femoral trochlea and assumes a load sharing ability with the patella.3-5 (Figure 6) The result is that as compressive loads continue to increase with further knee flexion, the patellofemoral reaction force tapers even as the joint reaction force continues to increase. With increasing knee flexion, the area of contact with the quadriceps tendon increases and at 90 degrees of flexion is estimated to be 1 to 2 times that of the patellar contact area and by140 degrees, 3 to 4 times that of the patellar contact area.3 Therefore it is important to note that the contact area on the patella near full extension is small and the area of contact at 90 degrees of flexion is large. Thus as the reaction force on the patellofemoral joint may be small near full extension, the articular cartilage pressure (force per unit area) may be relatively high and although the forces increase in deep flexion, the contact area has also increased resulting in pressures that could be less than those near extension. (Figure 7.) That said, Huberti noted that patellofemoral pressures are typically highest between 60-90 degrees of flexion.4 In addition to the increasing contact area on the proximal surface of the patella with increasing knee flexion up to 90 degrees, so does the thickness of the articular increase proximally on the patella.5 KINEMATICS: Constrained only by soft tissue tensions and contiguous boney and cartilaginous geometries, the patella is free to move about in-plane and out-of-plane displacements. Rotation about the transverse axis is flexion/extension; rotation about the anteriorposterior axis is spin; and rotation about the proximal/distal axis is tilt. Translations include medial/lateral, proximal/distal, and anterior/posterior. As the patella is positioned slightly proximal and lateral to the trochlea in full extension and the knee initiates flexion, the patella is directed medially into the trochlea and then as knee flexion progresses, the tracking assumes a gentle curve, concave facing lateral.3 At the same time, the patella rotates medially (tilt) guided by the medial retinaculum and then, once engaged within the trochlea (30 degrees) tilt is determined by the slope angle of the lateral trochlear facet.30 The patella can translate medially as much as 5 millimeters during initial flexion.3,31 The angle between the patellar tendon and the quadriceps tendon help to adjust the flexion/extension of the patella to maintain the patellar contact area perpendicular to the patellofemoral joint reaction force.32 Philippot et al20 have demonstrated the effects upon patellar tracking by the MPFL as well as the medial patellotibial ligament (MPTL) and the medial patellomeniscal ligament (MPML). As others have demonstrated, the MPFL is the primary restraint to lateral displacement of the patella during the initial 30 degrees of knee flexion as well as controlling tilt. But as the knee moves into deeper flexion (90 degrees), the MPTL and MPML are not inconsequential for the control of lateral shift and tilt. As noted, the MPFL is the primary restraint to lateral displacement of the patella during the initial 30 – 40 degrees of flexion (56%). 7,20,33 Beyond that, the slope of the lateral facet of the trochlea becomes the primary restraint, although the medial ligamentous structures continue to contribute (MPML/MPTL 28-48%, 45-90 degrees of flexion).20 It should be remembered, however, that the lateral retinaculum also contributes 10% restraint of lateral displacement.34 As with the retinaculum, the force vectors of the quadriceps have a posterior directed component as well as proximal. Studies have found that the vastus medialis influences patellar spin and tilt with less influence over translation.35,36 Furthermore, the orientation of the vastus medialis oblique is most efficient in greater degrees of knee flexion where it has been shown that the tension produced by the quadriceps and patellar tendon upon the trochlear geometry has the most influence in affecting patellar stability. EFFECTS OF PATELLOFEMORAL MALALIGNMENT: A number of studies have investigated the effects of patellar alignment and malalignment on patellofemoral contact pressures.3,37 In the normal knee, contact pressures are uniformly distributed across the medial and lateral facets from 20-90 degrees of flexion. Increasing the Q-angle, however gives non-uniform results. Huberti et al4 investigated the effects of the Q-angle on patellofemoral contact pressure. Following an increase of 10 degrees, half of the specimens studied shifted contact area to the lateral facet, unloading the medial facet. However, in the other half of specimens tested, contact area included the periphery of the medial facet as well as lateral facet with peak pressures found in both areas. In both scenarios, patellofemoral pressures were increased 45% at 20 degrees. Interestingly, decreasing the Q-angle from normal alignment also resulted in increased pressures with some specimens having the contact area shifted to the medial patellar facet while others had increased pressures found along the periphery of both medial and lateral facets. Bottom line, although changes in patellar alignment result in changes in patellofemoral contact areas and pressures, these changes are not uniform among knees.4,23,38 In addition to increased patellofemoral pressures with variations in tibial tuberosity position, Kuroda and others have shown the effects upon the tibiofemoral joint.22,27,39 Medialization, and especially over-medialization of the tibial tuberosity results in increased joint loading of the medial compartment as well as reducing pressures within the lateral compartment. Others have shown the reduction of control of lateral tibial rotation following medialization of the tibial tuberosity can be associated with increased pressures on the anterolateral surface of the lateral tibial plateau.22,40 Patellar malalignment can also take the form of an abnormal proximal position (alta) or an abnormal distal position (baja). There is sufficient information in the literature to understand that patella alta is associated with increased stress on the articular cartilage due to the relative decreased contact area available for joint loading at each degree of knee flexion.37,41,42 In addition, patella alta results in a delay of quadriceps tendon making contact with the trochlea (tendofemoral) therefore delaying the normal sharing of joint loading between the patella and the quadriceps tendon. Implications are for this condition to predispose to the development of anterior knee pain and patellofemoral arthrosis.28 Patella baja is commonly felt to be harmful to the patellofemoral joint. This condition is often associated with arthropathy, but most often associated with traumatic or posttraumatic conditions. In fact, studies have demonstrated that although the patellofemoral joint force can be increased with patella baja, the joint pressures are not increased due to the relative increased patellar contact area seen with the baja condition.29,43,44 Furthermore, the increased patellofemoral contact area as well as the earlier entry of tendofemoral load sharing may well protect the articular cartilage from increased stress. CONCLUSION: As Scott Dye has described, the human knee is the product of over 300 million years of evolution.45 The patellofemoral articulation remains a complex interplay of statics and dynamics between the surrounding ligaments, muscles and osseous geometries. Furthermore, although there are general consistencies of the mechanics and kinematics of the patellofemoral joint between individuals, common variations in osseous geometries and patellofemoral alignments can lead to even paradoxical effects of joint loading, both patellofemoral as well as femoral-tibial, following our surgical interventions. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. Wiberg G. Roentgenographic and anatomic studies on the patellofemoral joint. Acta Orthop Scand. 1941;12:319-409. Panni AS, Cerciello S, Maffulli N, Di Cesare M, Servien E, Neyret P. Patellar shape can be a predisposing factor in patellar instability. Knee Surg Sports Traumatol Arthrosc. Apr 2011;19(4):663-670. Schindler OS, Scott WN. Basic kinematics and biomechanics of the patellofemoral joint. Part 1: The native patella. Acta Orthop Belg. Aug 2011;77(4):421431. Huberti HH, Hayes WC. Patellofemoral contact pressures. The influence of qangle and tendofemoral contact. 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Reducing the lateral force acting on the patella does not consistently decrease patellofemoral pressures. Am J Sports Med. Jul-Aug 2004;32(5):1202-1208. Huberti HH, Hayes WC, Stone JL, Shybut GT. Force ratios in the quadriceps tendon and ligamentum patellae. J Orthop Res. 1984;2(1):49-54. Salsich GB, Ward SR, Terk MR, Powers CM. In vivo assessment of patellofemoral joint contact area in individuals who are pain free. Clin Orthop Relat Res. Dec 2003(417):277-284. Lee TQ, Sandusky MD, Adeli A, McMahon PJ. Effects of simulated vastus medialis strength variation on patellofemoral joint biomechanics in human cadaver knees. J Rehabil Res Dev. May-Jun 2002;39(3):429-438. Kuroda R, Kambic H, Valdevit A, Andrish JT. Articular cartilage contact pressure after tibial tuberosity transfer. A cadaveric study. Am J Sports Med. Jul-Aug 2001;29(4):403-409. Luyckx T, Didden K, Vandenneucker H, Labey L, Innocenti B, Bellemans J. Is there a biomechanical explanation for anterior knee pain in patients with patella alta?: influence of patellar height on patellofemoral contact force, contact area and contact pressure. J Bone Joint Surg Br. Mar 2009;91(3):344-350. Meyer SA, Brown TD, Pedersen DR, Albright JP. Retropatellar contact stress in simulated patella infera. Am J Knee Surg. Summer 1997;10(3):129-138. Feller JA, Amis AA, Andrish JT, Arendt EA, Erasmus PJ, Powers CM. Surgical biomechanics of the patellofemoral joint. Arthroscopy. May 2007;23(5):542-553. van Kampen A, Huiskes R. The three-dimensional tracking pattern of the human patella. J Orthop Res. May 1990;8(3):372-382. Krackow KA, Hungerford DS. Anatomy and kinematics of the normal knee. In: Hungerford DS, Krackow KA, Kenna RV, eds. Total Knee Arthroplasty. Baltimore: Williams & Wilkins; 1984. Senavongse W, Amis AA. The effects of articular, retinacular, or muscular deficiencies on patellofemoral joint stability: a biomechanical study in vitro. 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Q-angle influences tibiofemoral and patellofemoral kinematics. J Orthop Res. Sep 2001;19(5):834-840. Elias JJ, Carrino JA, Saranathan A, Guseila LM, Tanaka MJ, Cosgarea AJ. Variations in kinematics and function following patellar stabilization including tibial tuberosity realignment. Knee Surg Sports Traumatol Arthrosc. Oct 2014;22(10):2350-2356. Ward SR, Powers CM. The influence of patella alta on patellofemoral joint stress during normal and fast walking. Clin Biomech (Bristol, Avon). Dec 2004;19(10):1040-1047. Ward SR, Terk MR, Powers CM. Influence of patella alta on knee extensor mechanics. J Biomech. Dec 2005;38(12):2415-2422. Upadhyay N, Vollans SR, Seedhom BB, Soames RW. Effect of patellar tendon shortening on tracking of the patella. Am J Sports Med. Oct 2005;33(10):15651574. Bertollo N, Pelletier MH, Walsh WR. Relationship between patellar tendon shortening and in vitro kinematics in the ovine stifle joint. Proc Inst Mech Eng H. Apr 2013;227(4):438-447. Dye SF. An evolutionary perspective of the knee. J Bone Joint Surg Am. Sep 1987;69(7):976-983. LEGENDS: Figure 1. Wiberg has described three general morphotypes of the patella. A small more vertically oriented “odd facet” articulates with the trochlea in deep flexion. Figure 2. When viewed in the axial plane it is easier to understand the function of the medial and lateral retinaculum to engage the patella within the trochlea. Figure 3. The force distributions within the quadriceps tendon (FQ) and the patellar tendon (FL) are influenced by the knee flexion angle with greater force seen by the quadriceps tendon in flexion angles beyond 45 degrees of flexion and greater force within the patellar tendon between 20 - 45 degrees of flexion. (adapted from: Huberti, et al, J. Orthop Res. Vol 2, No 1, 1984) Figure 4. The patellofemoral reaction force (PRF) can be assessed by a vector analysis dependent upon the resultant vector of the quadriceps tendon strain force (QTF) and the patellar tendon strain force (PTF). The parallelogram of forces determined by the angle formed between the QTF and the PTF. Body position exerts a great effect upon the PRF by affecting the ground reaction force through the body’s center of gravity (CG) and the moment arms of the femur and tibia. (adapted from: Schindler and Scott, Acta Orthop Belg 2011, 77, 421-431) Figure 5. Contact areas of the patella and trochlea are not uniform and vary according to the range of motion. Patella contact (red) is initiated distally and traverses proximally as the knee falls into flexion. The green areas indicate the quadriceps tendon contact. (adapted from: Schindler and Scott, Acta Orthop Belg 2011, 77, 421-431) Figure 6. In the mid-range of knee flexion, the quadriceps tendon begins to turn around the distal femur and assumes a load-sharing function with the patella. Figure 7. Although patellofemoral force increases with knee flexion, increasing contact area also increase, including the load-sharing of the quadriceps tendon, resulting in modulation of patellofemoral pressure. (adapted from: Bellemans, The Knee 10 (2003) 123-126 Fig 1 Fig 2 Fig 3 Fig 4 Fig 5 Fig 6 Fig 7