Effect of Patellar Morphology on the Risk of Osteochondral Fracture after Patellar Dislocation: A Cross-sectional Study

Abstract

Objective: The risk of osteochondral fracture (OCF) after patellar dislocation has been shown to be related to patellofemoral anatomy, but its relationship to patellar morphology remains unknown. The aim of this study was to investigate the associations between patellar morphology and the risk of OCF after patellar dislocation.

Methods: A total of 140 patients with patellar dislocation between January 2018 and June 2023 were enrolled in this study and divided into two groups. Sixty-five patellar dislocation patients with OCF were included in the OCF group, while 75 patellar dislocation patients without OCF were included in the non-OCF group. Computed tomography was used to compare measurements of patellar morphology including Wiberg classification, patellar width and thickness, Wiberg angle, Wiberg index, facet ratio, lateral patellar facet angle, and patellar tilt angle. A logistic regression model was performed to evaluate the correlations between patellar morphology and the risk of OCF after patellar dislocation. Receiver operating characteristic curves were used to calculate the area under the curve (AUC) and determine the diagnostic values of patellar morphology for OCF after patellar dislocation. Subgroup analyses for gender and age were conducted to compare the differences in patellar morphology of PD patients.

Results: Wiberg angle was significantly lower in the OCF group (p = 0.017), while Wiberg index (p = 0.002) and facet ratio (p = 0.023) were significantly higher in the OCF group. According to the results of logistic regression analysis, Wiberg angle (odds ratio [OR] = 0.96, p = 0.022) and Wiberg index (OR = 1.105, p = 0.032) were the final relevant factors for the occurrence of OCF after patellar dislocation. The AUC was 0.622 (95% confidence interval [CI]: 0.529-0.714) for Wiberg angle, 0.65 (95% CI: 0.558-0.742) for Wiberg index, and 0.702 (95% CI: 0.615-0.788) for the combination of Wiberg angle plus Wiberg index.

Conclusion: Wiberg angle and Wiberg index were independent risk factors for the occurrence of osteochondral fracture after patellar dislocation. Moreover, Wiberg angle, Wiberg index, and the combination of Wiberg angle plus Wiberg index had good predictive diagnostic value for the occurrence of OCF after patellar dislocation.

Keywords: Computed tomography; Osteochondral fracture; Patellar dislocation; Patellar morphology; Patellofemoral instability.

FIGURE 1

Flow chart of patient selection.

FIGURE 2

Wiberg classification for patella morphology. (A) Type I: medial facet (concave) equal size as lateral facet (concave). (B) Type II: medial facet (flat, slightly convex) smaller than lateral facet. (C) Type III: medial facet (convex) very much smaller than lateral facet.

FIGURE 3

Measurements of the patellar morphology. (A) The patellar width: the length between the lateral (A) and medial edge (B) of the patella in the slide that showed the widest diameter of the patella. The patellar thickness: the length between the patellar front polar (C) and back polar (D). (B) Wiberg angle: the angle between the medial and lateral sides of the patella. (C) Wiberg index: the ratio of the transverse length of the lateral patellar facet (AE) to the patellar width (AB). (D) The facet ratio: the ratio of the length of the lateral side of the patella (a) to the length of the medial side (b). (E) Lateral patellar facet angle: the angle between the patellar transverse axis and the lateral sides of the patella. (F) Patellar tilt angle: the angle between the extension line of the maximum transverse diameter of the patella (line 1) and the tangent to the posterior condyles (line 2).

FIGURE 4

Receiver operating characteristic (ROC) curve analysis for Wiberg angle (WA), Wiberg index (WI), and combination of WA plus WI. ROC analysis showed that the area under the curve (AUC) was 0.622 (95% CI: 0.529–0.714) for WA, 0.65 (95% CI: 0.558–0.742) for WI, and 0.702 (95% CI: 0.615–0.788) for the combination of WA plus WI.

FIGURE 5

Preoperative magnetic resonance imaging and intraoperative arthroscopic findings of a 14‐year‐old patellar dislocation patient with osteochondral fracture. (A) Preoperative magnetic resonance imaging showed the fracture site on patella (black arrow). (B, C) The fracture site (B) and the fracture block covering the cartilage (C) was found under arthroscopy. (D–F) Preoperative computed tomography showed that the width of patella was 35.5 millimeters (D), Wiberg angle was 106.6 degrees (E) and Wiberg index was 0.65 (F).

FIGURE 6

Preoperative magnetic resonance imaging and intraoperative arthroscopic findings of a 21‐year‐old patellar dislocation patient without osteochondral fracture. (A) Preoperative magnetic resonance imaging showed that the medial patellofemoral ligament was injured. (B) The injury site on patella but no osteochondral fracture (white arrow). (C) There were no intra‐articular loose body. (D–F) Preoperative computed tomography showed that the width of patella was 47.9 millimeters (D), Wiberg angle was 133.6 degrees (E) and Wiberg index was 0.7 (F).