Does Relief from Intra-articular Anesthetic Injection Predict Outcome after Hip Arthroscopy?

OBJECTIVES: Intra-articular (IA) anesthetic injection is commonly performed as a diagnostic test in the setting of femoroacetabular impingement (FAI). Currently, there is a paucity of data correlating post-injection pain relief and outcomes after hip arthroscopy for FAI. The purpose of this study is to determine whether the amount of pain relief after IA injection predicts clinical and functional outcomes following hip arthroscopy. We hypothesize that increased pain relief (>50%) will correlate with better outcomes after surgery. METHODS: The records of patients undergoing hip arthroscopy for FAI at our institution between April 2007 and April 2012 were reviewed. We identified patients who underwent IA injection and subsequent hip arthroscopy. Inclusion criteria were: ultrasound or fluoroscopic guided intra-articular anesthetic injection performed at our institution, prospectively documented pre- and post-injection Numerical Rating Scale (NRS) pain scores, no prior ipsilateral hip surgery, and minimum 1 year follow-up. Pre-operative radiographs were reviewed and degree of osteoarthritis was determined using the Tonnis classification system. Outcomes were assessed with Modified Harris Hip Score (MHHS) and Hip Outcome Score (HOS). Univariate and multivariate models were performed to assess whether percent pain relief correlated with outcome scores. RESULTS: 99 hips in 96 patients met our inclusion criteria and included 71 females (74%) and 25 males (26%) with a mean age of 37.5 ± 14.0 years. 26 patients had Tonnis grade 0, 55 had grade 1, and 18 had grade 2 (0 grade 3). Mean pain relief after IA injection was 73.6 ± 36.1 (range 0-100) percent. 26 patients (26%) had ≤50% pain relief while 73 (74%) had >50% pain relief. Outcome scores were obtained at a mean 14.9 months. Mean MHHS, HOS-ADL, and HOS-Sport scores were 79.2 ± 17.3, 82.6 ± 17.3, and 67.4 ± 28.2, respectively. There was no statistical correlation between percent pain relief and MHHS, HOS-ADL, or HOS-Sport scores. There was no significant difference in outcome scores between those with ≤50% and >50% pain relief. While patients that received >50% pain relief by IA injection were more likely to achieve positive results (MHHS >70) with a likelihood ratio of 1.23 (95% CI 0.92-1.53), this was not significant. Multivariate regression analysis demonstrated no significant predictors of outcome, including age, gender, Tonnis grade, percent relief with IA injection, number of months postoperatively, or type of surgery. CONCLUSION: In patients undergoing hip arthroscopy for FAI, our data indicates that the amount of pain relief from IA injection is a poor predictor of short term outcome, even when adjusting for chondral degeneration. While anesthetic injections can be an important diagnostic tool in select patients, correlation of the clinical history, physical examination, and imaging findings are fundamental. In addition, outcome following hip arthroscopy remains multifactorial.