The "Outside-In" Lesion of Hip Impingement and the "Inside-Out" Lesion of Hip Dysplasia: Two Distinct Patterns of Acetabular Chondral Injury

Am J Sports Med. 2019 Oct;47(12):2978-2984. doi: 10.1177/0363546519871065. Epub 2019 Sep 6.


Background: Femoroacetabular impingement (FAI) and acetabular dysplasia lead to acetabular cartilage damage that commonly results in the chondral flaps seen during hip arthroscopy.

Purpose: To compare the acetabular chondral flap morphology seen during hip arthroscopy ("outside-in" vs "inside-out") with clinical and radiographic parameters underlying FAI and hip dysplasia.

Study design: Cohort study; Level of evidence, 3.

Methods: Patients who underwent hip arthroscopy by the senior author between 2013 and 2017 with a finding of Outerbridge grade IV acetabular chondral flap were included. Each procedure was retrospectively reviewed on video and chondral flaps were categorized as inside-out or outside-in. An inside-out designation was made for flaps exhibiting an intact chondrolabral junction with a detached sleeve of chondrolabral tissue from the central acetabulum, and an outside-in designation was made for centrally anchored flaps exhibiting a break in the chondrolabral junction. Radiographic markers of hip impingement/dysplasia were noted for each patient during assignment into 1 of 2 radiographic groups: group 1, lateral center edge angle (LCEA) >20 with FAI, and group 2, LCEA ≤20 with or without cam FAI. Associations between chondral flap morphology and clinical diagnosis were tested using a chi-square test.

Results: Overall, 95 patients (103 hips) were included (group 1, 78 hips; group 2, 25 hips). Among hips in group 2, 24 had concurrent cam FAI. There was a significant relationship between chondral flap type and radiographic diagnosis (P < .001). Among group 1 hips, 78% exhibited outside-in type chondral flaps, 12% exhibited combined outside-in and inside-out flaps, and 10% exhibited inside-out flaps. Group 2 hips showed 72% inside-out type chondral flaps, 16% combined, and 12% outside-in. Hips exhibiting outside-in type flaps were significantly more likely to be in group 1 (positive predictive value [PPV], 91%; negative predictive value [NPV], 69%). Similarly, hips exhibiting inside-out type flaps were significantly more likely to be in group 2 (PPV, 56%; NPV, 95%). Altogether, 90% of group 1 hips exhibited an outside-in lesion and 88% of group 2 hips exhibited an inside-out lesion.

Conclusion: Acetabular chondral flap type visualized during hip arthroscopy correlates with radiographic markers of hip impingement and hip instability. Outside-in flaps are highly predictive of FAI, whereas inside-out flaps are highly predictive of acetabular dysplasia.

Keywords: cartilage; chondral flap; femoroacetabular impingement; hip arthroscopy; hip dysplasia.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Acetabulum / diagnostic imaging
  • Acetabulum / pathology
  • Acetabulum / surgery
  • Adult
  • Arthroscopy / methods
  • Female
  • Femoracetabular Impingement / diagnostic imaging
  • Femoracetabular Impingement / pathology*
  • Femoracetabular Impingement / surgery
  • Hip Dislocation / diagnostic imaging
  • Hip Dislocation / pathology*
  • Hip Dislocation / surgery
  • Humans
  • Male
  • Radiography
  • Retrospective Studies