Two distinct groups of maltrackers (lateral and nonlateral) were distinguished based on PF LM displacement (Figs. 4, 5, 6). The discriminant analysis using PK4 and PK7 supported this definition of subgroups with 90.0% and 93.3% accuracy, respectively. Although the groups were divided based on LM displacement, the lateral group was 6.2 mm laterally displaced (p < 0.001), 7.4° laterally tilted (p = 0.032), and 6.8 mm superiorly displaced (p < 0.001), along with having increased slopes for lateral displacement (Δslope = 0.22, p < 0.001), lateral tilt (Δslope = 0.53, p < 0.001), and varus rotation (Δslope = −0.20, p = 0.013), compared with the nonlateral maltrackers (Figs. 5, 6). The lateral group differed from the asymptomatic group average for all PF value variables except PA displacement, whereas the nonlateral maltrackers differed from the asymptomatic group average for one variable only (increased PF flexion). As both groups had significant differences in 3D PF movement profiles from the asymptomatic average, the term “maltrackers” was maintained for both groups. Qualitatively, lateral tilt and displacement followed unique patterns in each maltracking group. For the lateral maltrackers, tilt and displacement had a small lateral change during early extension and an abrupt slope change at approximately 20° knee extension followed by a rapid lateral change in terminal extension. The nonlateral maltrackers had a small lateral change throughout extension. On average, the nonlateral group had less lateral hypermobility and fewer J-signs than the lateral group (Table 1).
Unlike the preliminary analysis, the subgroups for the larger cohort of patients with PF pain could not be defined accurately when using the three standard clinical measures (J-sign, Q-angle, lateral hypermobility) as predicting variables. Yet, a discriminant analysis using the continuous variables of Q-angle, lateral hypermobility, and quantitative J-sign could predict the groups to an accuracy of 90.6%. Thus, in general, the lateral maltrackers entered the study based on the presence of PF pain and at least one dynamic marker of maltracking (either patellar lateral hypermobility ≥ 10 mm, a positive quantitative J-sign, or both). This group had Q-angles greater and lesser than 15°. In general, the nonlateral maltrackers entered the study based solely on two factors: PF pain and a Q-angle of 15° or greater (static marker).
The clinical J-sign and the quantitative J-sign were not correlated with each other, but the quantitative J-sign was correlated with seven PF displacement and rotation variables (Table 2). Lateral hypermobility was correlated with the value of IS and the slope of PA displacement. Lateral hypermobility and LM displacement had the greatest influence in the discriminant analyses after all other factors were controlled for.
Clinical measures such as the Q-angle, lateral hypermobility, and the clinical J-sign have been used to infer excessive lateral PF displacement. Unfortunately, these measures have not been correlated to specific 3D PF displacement and rotations in past studies. Thus, the association between these clinical measures and 12 parameters of PF motion was investigated. In addition, past studies have suggested the presence of subgroups with unique PF maltracking patterns in patients diagnosed with PF pain syndrome. Identifying these subgroups may play a critical role in providing more effective interventions.
The knee motion evaluated in this study was limited to partial range of motion with no externally applied load. This did not limit the study conclusions, as the patella is less engaged with the femoral sulcus and maltracking patterns typically are most evident  in this range of motion . The required self-selected quadriceps activation during this free-kicking exercise likely further accentuated the maltracking patterns [5, 50]. Although insufficient power may have influenced the conclusions reached for certain variables, the between-group differences were small. Thus, the differences between these variables were likely clinically irrelevant. The clinical inclusion criteria were evaluated by one of the authors (KEA) for two knees with PF pain. A training session, held before data collection, ensured intraobserver and interobserver consistency. Removing these two knees from the analysis did not alter the final conclusions.
The finding that Q-angle is associated with medial and inferior PF displacement opposes the theory that a large Q-angle results in lateral subluxation [4, 10, 35]. Yet, in light of the work on the Q-angle's basic geometry [13, 17], the results are logical. According to Grelsamer et al. , a 2-cm lateral shift in the ASIS results in a 2° increase in Q-angle. Accordingly, a more medially placed patella and a shorter patellar tendon result in a larger Q-angle; specifically, the same 2-cm shift applied medially to the patellar center results in an 18.2° increase in Q-angle (assuming the distance between the patellar center and the tibial tuberosity = 7 cm ). Therefore, a large Q-angle is more indicative of a medialized patella than a lateralized ASIS. Applying this reasoning to the maltracking population, it can be theorized a large Q-angle is indicative of a medially placed patella under static conditions. This medial placement is a result of passive constraints alone because muscle activity is not present when the Q-angle is measured. For the nonlateral maltrackers (all but two of whom had a Q-angle ≥ 15°), these passive constraints are capable of counteracting the lateral pull of the extensors during active contraction and maintaining a medialized patella.
Establishing the presence of subgroups in the umbrella diagnosis of PF pain with maltracking likely will enhance treatment by allowing for more targeted interventions. For example, when a patient does not exhibit the classic PF maltracking patterns (excessive lateral tilt and displacement), pain often is assumed to be from overuse . Yet, PF pain may arise owing to alterations in the other PF displacements and rotations. For example, the nonlateral group had excessive PF flexion, which may decrease cartilage contact area, causing increased contact stress. In addition, the patella engages with the femoral sulcus further in terminal extension for this group, as compared with the lateral maltrackers (7-mm difference in PF superior displacement). Thus, the sulcus is more likely to provide the passive force, which maintains nearly normative axial plane movement. Excessive forces on the sulcus and patellar lateral edge are likely sources of pain. However, the lateral maltrackers had the classic PF maltracking patterns. In this group, the presence of patella alta reduces the overall PF contact area, causing increased stress and potentially PF pain . Patella alta creates a situation in which the patella leaves the sulcus groove earlier in the extension cycle, causing a loss of bony constraints and resulting in greater tilt and subluxation. Thus, a secondary pain source may arise from the high forces required to reengage the patella into the sulcus during flexion.
The fact that three different discriminant analyses had greater than 90% agreement with the definition of the maltracking subgroups provides strong evidence for the existence of these subgroups. In future studies, it may be possible to further divide the nonlateral maltrackers into three subgroups (medial, normal, lateral tilt) and the lateral maltrackers into two subgroups (normal, lateral tilt). The presence of five subgroups was not investigated owing to sample size restrictions. Being able to further subdivide the group likely will lend support to the three subgroups (lateral placement, lateral tilt, lateral placement with lateral tilt) established by Schutzer et al. .
Q-angle, lateral hypermobility, and the clinical J-sign may not be accurate gauges of lateral subluxation during dynamic activity. Distinct maltracking subgroups do exist in the patellar maltracking population. The quantitative J-sign is highly correlated with 3D PF displacements and rotations. Lateral hypermobility is a critical clinical variable in that it is different between maltracking subgroups, is correlated with PF motion, and has the greatest influence on the discriminant analysis. Work is ongoing to establish a larger database to explore finer details in these subgroups.
We thank Ching-yi Shieh, PhD, and Elizabeth K. Rasch, PT, PhD, for support on the statistical analysis and Steven Stanhope, PhD, for guidance throughout the project. We also thank Bonnie Damaska, Jamie Fraunhaffer, Jere McLucas, Dr. Barry Boden, and the Diagnostic Radiology Department at the National Institutes of Health for their support and research time.
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