Summary

Examination under anaesthetic of hips with FAI is a reliable method of defining underlying pathomorphology.

Abstract

Introduction.
Femoro-acetabular impingement (FAI) is a dynamic phenomenon. The decisions on FAI management are guided by radiological findings, most prominently MRI scan. However, MRI is suboptimal in assessing the precise dynamic nature of FAI, bony pathomorphology (CAM/pincer lesions) and, infrequently, labral pathology. There is paucity of knowledge on how informed our pre-operative planning is when based purely on MRI findings. We additionally aimed to evaluate the value of anterior acetabular sector angle (AASA) as a measure of anterior acetabular coverage assessment.

Materials.
We performed a review of a single high-volume surgeon’s cohort of patients that underwent hip arthroscopy for FAI. Pre-operative MRI findings were analysed, and correlated with examination under anaesthetic (EUA) and intra-operative arthroscopic findings (benchmark). The parameters analysed included alpha-angles versus presence of CAM lesions, precise nature of labral lesions, presence of pincer lesions and their correlation to AASA. Alpha-angle >50 degrees and AASA >65 degrees were deemed pathological.

Results.
We reviewed 150 patients who underwent 150 hip arthroscopies. There were 78 females and 72 males, average age 38 years (53-18). Intra-operatively, pincer was present in 20% of patients, CAM in 26%, and mixed impingement in 54%. MRI scans correctly identified the presence of pincer in 36% of cases, CAM lesions in 44%, and precise labral abnormalities in 80%. Although there was a statistically significant difference in reported AASA values between pure CAM-type and impingements involving the presence of pincer lesions (57o vs 63o p<0.05), this difference was absent between pure pincer and mixed impingement (62o vs 63o, p=0.62). Pre-operative EUA accurately identified the presence of CAM lesions in 89% and pincer-type lesions in 77%.

Conclusions.
Ability to precisely establish pathology enables thorough pre-operative planning. MRI scans, even when reported by experienced MSK radiologists, frequently fail to identify the impingement pattern. Pathological threshold of AASA >65 degrees in MRI-based axial-plane evaluation of pincer-type pathomorphology was shown to be unreliable. Therefore, we advocate pre-operative EUA to enhance the understanding of pathology, treatment planning and, ultimately, the success of hip arthroscopy.