Relationship of Baseline Physical Activity and Morphologic Factors in Patients With Lateral Patellar Instability

Patients

A retrospective analysis was conducted. Between December 2016 and December 2021, 82 consecutive patients underwent isolated medial patellofemoral ligament reconstruction, which was the first-line surgical treatment for patellar instability at our institution during this period. No patient underwent tibial tubercle osteotomy or other additional bony procedure. At our institution, surgical intervention is considered even after a first-time patellar dislocation when clinical instability and predisposing anatomic risk factors are present. The cohort also included patients referred after developing recurrent patellar instability following prior dislocation episodes managed elsewhere. Notably, during the study period, no eligible patients with patellar dislocation were managed nonoperatively. Exclusion criteria were (1) previous surgery on the index knee (n = 1), (2) concomitant anterior cruciate ligament reconstruction (n = 2), (3) congenital or habitual patellar dislocation (n = 3), and (4) patellofemoral osteoarthritis (n = 1). A total of 111 knees from 75 patients (18 male and 57 female; mean ± SD age, 20.4 ± 10.0 years) were included in this study. Of these, 34 patients were in the competitive sports group, 18 in the recreational sports group, and 23 in the nonsports group (Figure 1).

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Figure 1.

CONSORT (Consolidated Standards of Reporting Trials) diagram of patient inclusion in the study. MPFL, medial patellofemoral ligament.

Procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Declaration of Helsinki and its later amendments or comparable ethical standards. All patients provided written informed consent, and the study was conducted with approval from our institution's ethics committee.

Clinical Assessments

From medical records, age, sex, body mass index, laterality of knee joint, and Tegner activity score prior to symptom onset were collected. The interval from symptom onset to data collection had a median of 6 months (range, 0-74 months). Activity level was assessed based on patients’ self-reported physical activity prior to the initial patellar dislocation, using the Tegner activity scale as a reference. Patients were categorized into 3 groups according to their typical sports participation before the first dislocation: (1) competitive sports, defined as participation in organized competitive events or official matches; (2) recreational sports, defined as participation in noncompetitive sports activities or regular school physical education classes; and (3) nonsports, defined as no engagement in sports activities (Figure 1). Preinjury Tegner scores were obtained retrospectively from each patient based on his or her activity status before the initial patellar dislocation.

Image Evaluation

Predisposing morphologic factors were quantitatively assessed using preoperative radiographs and computed tomography (CT) scans. The following radiologic parameters were measured: (1) Insall-Salvati ratio (ISR) for patella alta^11,19 (Figure 2); (2) trochlear depth (TD) for trochlear dysplasia ⁸ (Figure 2); (3) congruence angle (CA) on Merchant view for lateral displacement of the patella ⁸ (Figure 3); (4) sulcus angle (SA) on Merchant view for trochlear groove depth ¹⁴ (Figure 3); (5) tilt angle (TA) on Merchant view for lateral tilt of the patella relative to the femoral condyles ³ (Figure 3); and (6) tibial tuberosity–trochlear groove (TT-TG) distance on superimposed CT images for lateral malposition of the tibial tuberosity ⁸ (Figure 4). In addition, trochlear dysplasia was classified according to the Dejour classification using true lateral radiographs, based on the presence of the crossing sign, supratrochlear spur, and double contour sign.^6,7 All radiologic assessments were performed on a digital radiography system.

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Figure 2.

Measurement of the Insall-Salvati ratio (ISR) and trochlear depth (TD). The ISR was calculated as the ratio of the patellar tendon length (LT), measured from the inferior pole of the patella to its tibial insertion, to patellar length (LP), measured along its greatest diagonal (from the superior to the inferior pole). TD was defined as the distance from the trochlear floor (B) to the most anterior condylar contour line (A), measured along a line subtended 15° distal from the perpendicular intersecting the tangent of the posterior femoral cortex and the most proximal portion of the posterior femoral condyles.

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Figure 3.

Measurement of the sulcus angle (SA), congruence angle (CA), and tilt angle (TA). The SA was defined as the angle formed by lines connecting the highest points of the medial and lateral femoral condyles to the deepest point of the trochlear sulcus (AOB). The CA was defined as the angle between the line bisecting the SA and the line connecting the sulcus apex to the central ridge of the patella (COD). The TA was defined as the angle between the line connecting the anterior edges of the femoral condyles and the line spanning the maximal width of the patella (E).

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Figure 4.

Measurement of the tibial tuberosity–trochlear groove (TT-TG) distance using superimposed femoral and tibial computed tomography images. The TT-TG distance was defined as the separation between 2 parallel lines (yellow): one passing through the center of the tibial tuberosity and the other through the base of the trochlear groove (green), both perpendicular to the posterior condylar line (red).

Statistical Analysis

Patients were compared among 3 groups (competitive sports, recreational sports, and nonsports). Demographic variables compared using 1-way analysis of variance and chi-square test. In the analysis of morphological parameters, because some patients contributed bilateral measurements, within-patient correlation was addressed by clustering at the patient level. Continuous outcomes were analyzed using generalized estimating equations with a Gaussian distribution and an exchangeable correlation structure. Overall group effects were assessed using Wald chi-square tests, with pairwise comparisons conducted using Wald tests based on robust standard errors. For the categorical outcome (Dejour classification), multinomial logistic regression with cluster-robust standard errors was used. Overall and pairwise group differences were evaluated using Wald tests. The Holm step-down procedure was applied to adjust for multiple comparisons. All tests were 2-sided, with statistical significance set at P < .05.

Intraclass Correlation Coefficients

All bone morphology measurements were performed by knee surgeons with >10 years of clinical experience. For interobserver reliability, 2 surgeons (Y.K. and E.S.) independently performed blinded measurements at separate time points. For intraobserver reliability, 1 surgeon (Y.K.) performed duplicate measurements at separate time points.

Limitations

This study has several limitations. First, the groups differed significantly in age, sex distribution, and likely skeletal maturity; the competitive sports group tended to be younger and predominantly male, whereas the nonsports group tended to be older and predominantly female. These differences may confound the observed associations and should be considered when interpreting the findings. Second, this study did not distinguish between patients with first-time and recurrent patellar instability. Its retrospective design also limited the establishment of clear and definitive diagnostic criteria. Although recorded diagnoses were assumed accurate, this assumption introduces potential bias. Differentiating first-time dislocation from recurrent patellar instability is often challenging, as patients who report “instability” may in fact be experiencing repeated episodes of dislocation or subluxation. Furthermore, because our institution primarily receives referred patients, initial symptom onset and early clinical history in some cases occurred prior to presentation at our hospital. Accordingly, certain historical information may have been based on prior records and patient recall rather than contemporaneous documentation at symptom onset. This may introduce recall or information bias and should be considered when interpreting the findings. Third, the study compared imaging findings with patients’ self-reported sports participation but did not assess objective measures of physical function or sports performance. A further limitation is the use of the ISR to evaluate patellar height. Although ISR is a widely used method in patellar instability research and is minimally affected by knee flexion when images are obtained at ≥30° of flexion, it can still be influenced by patellar tendon length. Moreover, multiple methods exist for assessing patellar height, each with its own advantages and limitations, and no single technique is universally superior. ¹⁹ These factors may have introduced measurement bias, and future studies incorporating multiple patellar height indices may provide a more comprehensive assessment. Fourth, this study included only patients who underwent surgery performed at our hospital. Because our institution functions primarily as a referral center, all patients evaluated for patellar instability during the study period underwent surgical treatment. Patients treated nonoperatively at other institutions were not captured in this cohort. Therefore, external validity was not assessed, and the findings may not be generalizable to all populations or clinical settings. Finally, although clustering methods were used to account for within-patient correlation, the inclusion of both unilateral and bilateral cases may represent a structural limitation inherent to the study design.