Patella subluxation is associated with PF OA progression. Medial displacement of the patella predisposes to medial JSN progression, whilst increasing lateral displacement predisposes to lateral JSN progression. Patella tilt is also associated with disease progression and a trend to increasing pain severity.
The most common symptoms of knee OA occur with activities that preferentially engage the PF joint, and generate the highest forces and torques, such as descending stairs, and arising from a chair (32
). These activities load all compartments of the knee; however, the forces are greatest in the patellofemoral joint. Patellar tilt appears to predispose to pain presumably through an aberrant dispersion of the forces through the PF joint. Specifically Grelsamer, et al (16
) hypothesized that if the patella is tilted painful stresses can develop (16
Each measure provides different information about the exposure of patellofemoral malalignment. The measurement of bisect offset provided strong effects as an exposure for both medial and lateral PF JSN progression. In contrast measurement of tilt and sulcus angle appeared to influence the risk of medial progression more than lateral progression. One potential explanation for this is that medial PF JSN progression was more common than lateral providing greater opportunity to assess the effect of these exposures on this outcome.
The etiopathogenesis of osteoarthritis is widely believed to be the result of local mechanical factors acting within the context of systemic susceptibility. OA is characterized by changes in structure and function of the joint with the central component being degradation and subsequent loss of articular cartilage with related changes in the underlying bone (35
). Patellae that are located centrally in the trochlear groove and not malaligned are thought to be less likely to develop osteoarthritis (14
). When the cartilage in a compartment is under stress due to the aberrant biomechanics of the patella, there is an increased risk of JSN, while when it is unloaded JSN is less likely to occur.
The potential relation of alterations in patellar alignment predisposing to symptoms has been explored in patellofemoral pain syndrome; a condition that typically manifests in a much younger spectrum of the population than PF OA. Previous studies in persons with PF pain syndrome have identified abnormalities in both kinematics and joint stress (36
). More recently there have been suggestions that one of the long term sequelae of PF pain syndrome may be PF OA warranting joint replacement (39
To appreciate the forces that potentially can lead to increased patellofemoral loading it is helpful to consider the unique biomechanics of this joint; thus providing insights to risk factors for PF OA (15
). The patella increases the mechanical advantage of extensor muscles by transmitting forces across the knee at greater distance (moment) from the axis of rotation. In so doing, it increases the functional lever arm of quadriceps as well as changing the direction of pull of the quadriceps mechanism. It is the principal site of insertion of quadriceps, and it transmits the tensile forces generated by the quadriceps to the patellar ligament. Stability of the patellofemoral joint is dependent on the passive, dynamic and static restraints around the knee. The primary dynamic restraint is the quadriceps muscles. The primary static constraint is the articular anatomy of the femoral condyles in particular the trochlear depth/ sulcus angle and the shape of the retropatellar surface (40
The dynamic and static stabilizing forces of the patella cause the patella to compress (joint reaction force) against the femur. This is termed the patellofemoral joint reaction force (PFJR) and this force increases with increasing knee flexion. The joint reaction force during walking (10–15 degrees of flexion) is approximately 50% of bodyweight. Walking up stairs (60 degrees) the JRF is 3.3*bodyweight. During squats (130 degrees), the JRF is 7.8*bodyweight (17
). Previous studies have suggested that patellofemoral joint kinematics and mechanics may directly contribute to PF OA (19
The three measures patella alignment assessed on the skyline radiographs are those most frequently cited in the literature (16
). In the mid 1990’s, Harrison and colleagues examined skyline view knee x-rays of 109 knees in 65 patients with symptomatic patellofemoral osteoarthritis. They found that the amount (defined as summary score of radiographic changes, i.e., joint space loss, osteophyte formation, and bone cystic changes, in the patellofemoral joint) and site of patellofemoral arthrosis were correlated with patella position and limb alignment. Patellae that were located centrally in the trochlear grove had the lowest radiographic score for arthritis. Subluxation of the patella either medially or laterally was associated with an increased risk for radiographic scores (19
). The findings of this study suggest that patella malalignment appeared to be associated with the severity of overall radiographic changes in patellofemoral osteoarthritis patients.
Our results add to this by demonstrating that patella malalignment predisposes to progression. The few studies that have explored PF OA show that the lateral PF compartment is more frequently affected than the medial (11
). Previous studies have shown that the patellofemoral joint reaction force (PFJR) may directly contribute to patellofemoral osteoarthritis (41
). One parameter that may influence the PFJR is patella malalignment. The lateral compartment likely has higher contact pressures as a result of increased patella tilt and lateral subluxation (14
). This may be why an application of a medial glide to the patella (forcing the patella medially and away from an overloaded lateral compartment) in previous studies of taping leads to a positive effect on knee symptoms (44
). The potential for therapies that realign the patella to have a structure modifying effect has not been explored.
Our findings have a number of potential limitations that warrant consideration. One concern is the potential for collinearity in our analyses. For example lateral patellofemoral JSN itself allows increased lateral subluxation through alteration in the structure of the PF joint. In the absence of observational studies of longer duration with more time-points the precise sequence of events awaits further exploration. Further the number of cases particularly in the extreme quartiles was often small hence the often wide confidence intervals. We would commend replicating this analysis in a larger dataset.
The self reported assessment of pain in the knee is for the knee as a whole. Knee symptoms can emanate from a number of different tissues including the subchondral bone, synovium, retinaculum, skin, muscle and nerves. In addition it can come from the patellofemoral joint or the tibiofemoral joint. Assessment of the alignment of the patellofemoral joint provides only one small keyhole view into a complex array of possible sources of knee pain. Further this study suggests that if a relationship exists between patella alignment and pain it is weak.
We studied elderly subjects who were well-functioning one year prior to the baseline OA assessments. Our findings may not apply to other groups. The population was older at the age of inception than many other OA cohorts. A distinct advantage of this sample is the inclusion of a large sample of blacks however this may detract from the ability to compare the findings directly with other predominantly Caucasian cohorts such as Framingham. A large proportion of those who attended the baseline knee x-ray exam did not attend for follow-up x-ray exam. Those who did attend for follow up tended to have more OA and more symptoms thus representing a more severe OA group than the whole sample. This may have afforded us greater power to detect the associations we did find.
The selection methods ensured that at the baseline examination of the parent study, eligible participants were free of disability in activities of daily living and free of functional limitations (defined as difficulty walking a quarter of a mile or up 10 steps). This may have biased the sample selection at the year 1 follow-up exam for the present study such that those with the most severe knee OA may have been excluded. However this population has been used in other analyses which suggests that the pattern of OA is similar to that in other cohorts (22
). Similarly the control sample (those free of knee symptoms at baseline) were limited in number in comparison with the true prevalence of those without knee symptoms in the community. The analyses presented in this manuscript pertain to the relation between patella alignment and PF OA. If there were a bias it would be towards not finding a relationship between patella alignment and PF OA as more disabled (and potentially with that those with the most severe OA/ most progression) would have been excluded. In addition, the contrast between a small asymptomatic sample and those with symptoms may have reduced our opportunity to find an association between patella alignment and pain.
The WOMAC scale was not normally distributed (25% subjects had WOMAC pain score as zero) and despite trying standard methods to normalize this distribution we could not make this more normally distributed. This said linear regression is robust to violation of assumptions of normality.
The x-rays taken in our study were not acquired dynamically and were taken at one knee flexion angle. A laterally displaced patella (higher BO) and/or lateral border of patella tilted down on these images likely indicates the tightness of the structures that hold patella in a lateral position (lateral retinaculum, vastus lateralis, ITB). In this situation, during dynamic movement of the knee greater load and therefore greater shear force will be placed on the lateral PF compartment, in comparison to the situation where patella is located directly against trochlear sulcus and the load forces are divided equally between lateral and medial PF compartments. A static view of the knee taken at one flexion angle is likely to underestimate the true exposure to patella malalignment. Further evaluation using dynamic imaging techniques such as MRI would allow further evaluation of this exposure. For all films there was a single reader who was unblinded to sequence in reading alignment and the individual radiographic features. This may have biased the reader in detecting and reading progression.
In sum we have found that patella subluxation, sulcus angle and patella tilt are associated with PF OA progression. These findings warrant replication in other datasets. Assuming they are replicated, and these findings reinforce the importance of PF alignment both for symptoms and PF OA progression, this has important implications. PF alignment is potentially modifiable through footwear advice (47
), taping and/ or knee bracing (45