Paper 53: A Predictive Model for Mobile vs. Immobile Osteochondritis Dissecans Lesions of the Knee

Objectives: Osteochondritis dissecans (OCD) of the knee is a focal idiopathic alteration of subchondral bone and/or its precursor with risk for instability and disruption of adjacent cartilage that may result in premature osteoarthritis. Treatment options vary depending on multiple patient and lesion characteristics, including lesion mobility. Recent multicenter studies helped develop the ROCK (Research on Osteochondritis Dissecans of the Knee) Arthroscopy Classification of OCD lesions of the knee. To advance the clinical utility of the classification system, this current investigation aimed to develop a simple, clinical outcomes-based model that could be used to differentiate lesion mobility. We hypothesized that a multivariable model including patient demographics and physical examination findings at clinical presentation would predict the arthroscopically confirmed mobility status of the OCD lesion with high sensitivity and specificity. Methods: Patient data for the predictive modeling came from a multi-center national prospective cohort of patients with osteochondritis dissecans of the knee. Inclusion criteria included patients < 19 years of age, operatively treated OCD of the medial or lateral femoral condyle, and appropriate preoperative and intra-operative data. Demographic, preoperative physical examination, and radiographic data were considered independent predictors in the model. Multivariable logistic regression analysis using stepwise model selection was used to determine factors associated with the likelihood of a mobile versus an immobile lesion. Receiver operating characteristic (ROC) curve analyses were used to dichotomize relevant variables for a final predictive model. The authors reviewed the relevant variables for clinical utility. A 75% partition of the data was used for model training, and 25% was held out for model validation testing. Quantitative model fit statistics were computed using the hold-out data, including sensitivity, specificity, and the area under the ROC curve (AUC), along with the corresponding 95% confidence interval (CI). Results: 479 patients in the prospective cohort who were less than 19 years of age and had surgical treatment of a knee OCD lesion were reviewed. 407 patients had usable preoperative and intra- operative data, including 235 immobile and 172 mobile lesions. Mean chronologic age of the 407 patients was 13.7 (2.2 SD) years with 62% male. Mean height was 63.6 (5.3 SD) inches and 129.5 (40.4 SD) lbs. Mean BMI was 22 (5.1 SD). 21% of the total cohort presented with an effusion on physical examination and 20% presented with reduced range of motion either in flexion or extension compared with their contralateral knee. Patient demographics (age, sex, age, height, weight, BMI) and physical examination findings of relative loss of motion (ROM) compared to unaffected knee and presence of effusion on examination were primary variables of interest. 88 patients were excluded because of missing partial data for modeling. 319 patients had complete data for modeling (training data 75%; n=239; validation data 25%; n=80). Using the training dataset, the multivariable analysis found that chronologic age ≥ 14 years, effusion on physical examination (p=0.004), and any loss of ROM on physical examination (p=0.004) increased the likelihood of mobile lesion controlling for male sex (p=0.42) and weight >120lbs (p=0.23). In the 25% holdout sample (n=80), this model yielded a sensitivity of 83%, a specificity of 75%, and an AUC of 0.86 (95% CI, 0.78-0.94). The final predictive model was created from dichotomized relevant variables (Figure 1). Conclusions: This predictive model for knee OCD lesions indicates a patient presenting with chronologic age ≥ 14 years, evidence of a knee effusion on physical examination, and reduced ROM relative to the contralateral knee is between 90% and 95% likely to have a mobile lesion compared to an immobile lesion using the ROCK Arthroscopy Classification. The ability to predict the mobility of knee OCD lesions before surgical intervention can facilitate improved surgical planning and more relevant communication with families regarding the procedures that will be required. Specifically, this can aid in distinguishing between less invasive interventions such as drilling versus more invasive procedures such as OCD fixation and salvage.