Patella height measurements in patients with patellofemoral instability: the power to predict patella height indices.

INTRODUCTION: Numerous indices for patella height measurement are described in the literature; however, there is not a universally accepted 'gold standard'. Choice of indices varies depending on personal preference and previous experience, making it difficult to compare results between studies. The aims of this study were to assess the correlation between different patella height indices and generate statistically derived formulae that can be used to predict the value of one patella height ratio when another is known. MATERIALS AND METHODS: Patients with patellofemoral instability were radiologically evaluated using both X-ray and Magnetic Resonance Imaging (MRI) of the knee. Two observers measured six validated patella height indices at two separate time intervals. All measurements were aggregated together and a mean average for each patella height ratio was calculated. These results were then statistically analysed using Pearson product-moment correlation and linear regression tests. RESULTS: Forty-four patients were included in the study. A statistically significant correlation was found between the majority of patella height ratios. On X-ray, the strongest correlation (r = 0.92, p < 0.001) and regression (R2 = 0.85, p < 0.001) coefficients were between the Blackburne-Peel and Caton-Dechamps ratios, with the formula; CD = 0.20 + (0.95xBP). On MRI, the strongest correlation (r = - 0.84, p < 0.001) and regression (R2 = 0.71, p < 0.001) coefficients were between the Patellophyseal index and Patellotrochlear index, with the formula; PP = 1.02-(1.10xPT). CONCLUSION: The formulae produced in this study can predict the value of one patella height ratio when the results of another are known. This could facilitate systematic reviews and meta-analyses by allowing the pooling together of data from many studies.

Inter- and intra-observer reliability of patellar height measurements in patients with and without patellar instability on plain radiographs and magnetic resonance imaging.

INTRODUCTION: Many patella height indices exist in the literature. There is no single universally accepted radiological assessment for measuring patella height. The aims of this study were to determine which of the commonly used indices can most reliably measure patella height and compare the findings on both plain X-ray and magnetic resonance imaging (MRI) of the knee. METHODS: This case-controlled study compared patients with recurrent patella instability (patella group) to a control group. Two observers measured six validated patella height indices on X-ray and MRI of both groups at two separate time periods. Between-group and within-group statistical analysis was undertaken of the data. The inter- and intra-observer reliability was assessed using the intraclass correlation coefficient (ICC) and the kappa measure of agreement (k). RESULTS: Forty-four patients comprised the patella group and 50 patients the control group. There was a significant difference of most indices between the two groups (p < 0.05). There was a significant difference of most indices between the two imaging modalities (p < 0.05). The Insall-Salvati ratio had the greatest inter-observer reliability for both X-ray and MRI (ICC = 0.79 to 0.97; p < 0.001) (k = 0.50 to 1.00; p < 0.001). CONCLUSION: Patella height indices significantly differ when measured on X-ray as compared to MRI. This may infer that a different set of normative values are required for each radiological modality, which we have proposed in this study. Overall, the Insall-Salvati ratio performed best and shows a high degree of intra- and inter-observer reliability on both X-ray and MRI.

Inter- and intra-rater reliability of knee flexion angle measurements on X-ray and MRI.

Background: Range of motion (ROM) is an important aspect of orthopaedic patient assessment. It can be measured at the knee joint by determining the knee flexion angle (KFA) a patient can achieve at extremes of flexion and extension. As with any measurement, the accuracy and reliability of the method used determine its validity. The consistency of magnetic resonance imaging (MRI) scans as compared to the current gold standard of X-ray remains unknown in terms of KFA evaluation. The aim of this study was to assess and compare the reliability of measuring KFA between X-ray and MRI scans. Methods: This study included 80 patients (94 knees) who had attended a specialist knee clinic due to varying knee pathologies and undergone both X-ray and MRI scans. Lateral and T1-weighted sagittal imaging views (respectively) were used to measure KFA by two trained observers independently at two separate time points, 8 weeks apart. The data was then statistically analysed and intra- and inter-observer reliability calculated using the intraclass correlation coefficient (ICC). Results: The intra-observer reliability for X-ray was 0.96 (P<0.001) and that for MRI was 0.83 (P<0.001). The inter-observer reliability for X-ray was 0.99 (P<0.001) and that for MRI was 0.81 (P<0.001). All the intra-class correlation coefficients were graded as excellent in both the intra- and inter-observer reliability analysis. Overall, the mean KFA was notably higher on X-ray measurements than that on MRI scans. There was a statistically significant difference between Time 1 and Time 2 measurements (17.7° vs. 16.8°) for MRI data (P=0.022). No significant difference was found for X-ray measurements (46.4° vs. 45.6°) in this regard (P=0.182). Conclusions: Both X-ray and MRI allow KFA to be measured with an excellent degree of reliability. However, X-ray measurements were overall superior to that of MRI mainly due to the larger field of view of the visible on-screen image which more readily identifies the anatomical landmarks required to measure KFA.