Abstract
Introduction
It is not known if there are side-to-side differences in knee morphology in patients with bilateral patellar instability (BPI). Such knowledge would help to answer the question whether similar treatment and prognosis could be expected for both knees in patients with BPI. The purpose of our study was to evaluate and compare anatomic / morphologic risk factors between knees in patients with BPI.
Materials and Methods
Of 294 patients who underwent surgical stabilization for patellar instability (2008-2017), 32 patients were identified who underwent staged or simultaneous bilateral surgery. This formed the study cohort. Demographic data, mechanism of initial injury and first knee side to be symptomatic were recorded. Radiographs and MRI of these 64 knees were evaluated for trochlear dysplasia (trochlear depth), patellar height (Caton-Deschamps index, Insall-Salvati ratio), patellar tilt, TT-TG distance, sulcus angle (cartilaginous and bony) and trochlear bump (cartilaginous and bony). The number and percentage of knees with pathologic values for measured risk factors were analyzed. For each risk factor, 15% side-to-side differences were considered significant. Statistical correlation was calculated between the time of presentation (first symptomatic knee) and magnitude of risk factors. All measurements were repeated after a 3-week interval and ICC was calculated to determine intraobserver reliability.
Results
The mean age of 32 patients was 14.6 years. 17/32 patients were females. 29 patients had staged surgery with mean interval of 21 months between surgeries; 3 patients had simultaneous surgery on each knee. Patients with BPI had multiple risk factors. 22/32 patients (69%) had the same Dejour type of trochlear dysplasia. For each measured risk factor, the number and percentage of patients with 15% or more side-to-side differences is shown (Table 1). There were significant side-to-side differences in patellar height ratios, TT-TG distance, and trochlear bump. The timing of presentation did not correlate with increased risk factors, i.e, the knee that presented earlier did not have increased risk factors compared to contralateral knee. ICC showed excellent intraobserver reliability.
Conclusion
There were significant side-to-side differences between knees in patients with BPI. Treatment and prognosis should be based on anatomic risk factors for each knee in patients with BPI.
| Risk Factor | Caton-Deschamps | Insall-Salvati | Patellar Tilt | TT-TG distance | Bony bump | Cartilage bump | Trochlear depth |
|---|---|---|---|---|---|---|---|
| Pathologic value | =/>1.2 | =/>1.5 | =/>20* | =/>20 mm | =/>3 | =/>6.9 | =/<3 mm |
| Mean (SD) for all knees | 1.45 (0.3) | 1.7 (0.3) | 20.8“ (8.7) | 15.9 mm (5.1) | 3.1 mm (2.0) | 5.5 mm (2.0) | 0.88 mm (1.8) |
| No (%) of knees with pathologic value | 52 (81%) | 51 (80%) | 31 (48%) | 11 (17%) | 29 (45%) | 12 (19%) | 60 (94%) |
| Range of difference (%) between knees | 0-0.49 (0.1-41%) | 0.01-0.72 (0.6-42.9%) | 0-16* (0-100%) | 0.1-12.4 mm (0.6-98.4%) | 0-3.5 mm (2.9-100%) | 0.1-3.7 mm (1.6-533%) | 0-3.85 mm (0-1700%) |
| No (%) of patients with >15% side-to-side diff | 11 (34%) | 9 (28%) | 16 (50%) | 22 (69%) | 20 (63%) | 19 (59%) | 27 (84%) |