chapter 2: Biomechanics of the Patellofemoral Joint
Patellofemoral Contact Areas
The absolute PFJR is only one part of the equation in understanding the mechanics of the patellofemoral joint in health and disease. The second essential part is to understand what is happening to the patellofemoral contact area. Although the PFJR is steadily increasing with increasing flexion, so is the patellofemoral contact area. Under normal loading conditions, this is not sufficient to maintain a constant unit load, but it is helpful.
Wiberg's classic article (26) on the patellofemoral joint contains a great deal of information concerning the contact areas, which seems to have been lost by many subsequent authors. His technique of serially sectioning cadaveric knees frozen in various degrees of flexion posed three problems: (1) only one position could be used per joint, (2) the patellofemoral joint was not loaded, and (3) the method necessitated reconstruction of the contact area from the serial sections, making them difficult to visualize. The essentials of his work, however, have been confirmed.
Goodfellow et al (27) reported a method that allows multiple contact prints per cadaver knee under a load qualitatively but not quantitatively simulating normal knee flexion (Fig. 2.5, A). In this technique, the area of articular contact was delineated by dyeing noncontact cartilage using a technique described by Greenwald and Haynes (28) and modified by Deane (29). This model demonstrated that after 30° flexion and under static conditions, patellar stability is determined by the resultant of force perpendicular to the articular surfaces. The congruence of the surfaces in all degrees of flexion contributes to stability under load in the same way that Hsieh and Walker (30) have shown for the tibiofemoral joint. To come out of contact, the patella has to work "uphill" on the trochlea against the resultant of force applied by the quadriceps and patellar tendon forces.
The patella first begins to glide onto the articular surface of the trochlea at approximately 10° of knee flexion. The controlling factor is the length of the patellar tendon. In patella alta, greater flexion will be necessary before the patella reaches the relatively stable seating of the trochlea. As the patella first begins to glide onto the articular cartilage of the trochlea, the contact print shows why the transition from lateral femoral condyle to femoral metaphysis is smooth, whereas the medial condyle presents a definite step of varying degrees of prominence. Pressure from the patella molds the lateral femur, whereas the median ridge does not come in contact with the patella under the usual circumstances. The 20° contact print (Fig. 2.5, B) shows the smooth zone of contact extending from near the secondary ridge between the medial and lateral facets to near the lateral border of the patella. The continuous nature of this contact encompassing the medial and lateral facets is characteristic of the contact up to 90° of flexion. Although one can speak of separate medial and lateral facets, there is not a corresponding separate zone of contact for each facet, but rather a band of contact on the patella that moves proximally with increasing knee flexion (Fig. 2.5, C). In 1995, Singerman (31) confirmed this pattern of proximal patellar load‑bearing shift with increasing knee flexion. In normal alignment, the lateral to medial contact area ratio is 1.6 : 1.0, with mean pressure the same on both facets (32).
The contact zone reaches the proximal patellar border by 90° flexion, with the contact area increasing as the contact zone moves proximally and as knee flexion proceeds (Fig. 2.5, D). Thus, in the first 90° of flexion, virtually all of the patellar articular cartilage, with the exception of the odd facet, is brought into load‑bearing contact with femoral articular cartilage. As this contact band has been moving up the patella, it has also been steadily increasing in area (Figs. 2.5, E‑G). Considering the earlier demonstration of the increasing load with progressive flexion, this would appear to be a sophisticated mechanism for maintaining a relatively constant unit load. Figure 2.6 shows area comparisons for varying degrees of flexion for four knees in which these measurements were carried out. Aglietti and co‑workers (33) found a similar quantitative increase in patellofemoral contact area with increasing flexion.
By 135° of flexion, the contact pattern has changed drastically. The central ridge and medial facet now lie free in the intercondylar notch, completely out of contact (Figs. 2.6 and 2.7). The entire odd facet engages the lateral border of the medial condyle. This area of the medial condyle contacts the medial tibial spine in full extension, and it is also the classic site for osteochondritis dissecans (6, 22, 34). The lateral patellar facet covers a large part of the lateral condyle of the femur, whereas the medial condyle is not covered. It should be noted that the lateral contact area on the femur corresponds to the tibial contact area on the femur with the knee in full extension. Eckstein et al (35) have noted that subchondral bone density maxima correspond to this part of the patella, being greatest at the proximal lateral facet.
In passing from the 90° contact pattern to the 135° pattern, the zone of contact has passed over the ridge separating the medial and odd facets (36). Patellofemoral contact has been characterized up to 90° of flexion by congruence between the patellar and trochlear facets. Even the contact between the odd facet and the lateral border of the medial condyle is normally characterized by congruence grossly, but Fujikawa et al (37) noted that the femoral trochlea angle was slightly greater than the patellar facet angle at every region of contact between patella and femur. However, in passing from 90 to 135°, the convex ridge of cartilage, which is normally thick, comes into apposition with convex femoral articular cartilage, which suggests the possibility of high unit load. This transition takes place, however, in the proximal patella where the ridge is normally not so prominent. This may account for the fact that not every patella shows pathology in this area. Also, retropatellar contact stress is reduced past 70° of knee flexion because of direct force transmission to the patellar tendon (32).
From an observation of the contact prints alone, one could come to the conclusion that the area available for load‑bearing decreases after 90° of flexion. However, a look at the femur after 90° flexion shows a broad band of unstained cartilage proximal to the site of patellar contact (Figs. 2.5, C‑E). After 90° flexion, the posterior surface of the quadriceps tendon is brought into contact with the trochlear facets of the femur. Goymann and Mueller (15) have referred to this as the "turn‑around" of forces. Once the quadriceps tendon comes into a load‑bearing relationship with the femur, the actual compression forces are divided between the extensive "tendofemoral" contact and the patellofemoral contact. This would appear to be another sophisticated means of maintaining constant unit load in situations where total load is increasing.
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