Is Subchondral Acetabular Edema or Cystic Change on MRI a Contraindication for Hip Arthroscopy in Patients With Femoroacetabular Impingement?

Abstract

Background:

The outcome for arthroscopic treatment of femoroacetabular impingement (FAI) can worsen with increasing arthritis. However, there remains a subset of hips with relatively maintained joint space but with acetabular subchondral edema and cystic change with unknown outcome on magnetic resonance imaging (MRI).

Purpose:

(1) To correlate MRI findings of subchondral acetabular edema/cystic change with arthroscopy grading of articular cartilage and (2) to determine whether postoperative outcome was worse for patients with subchondral edema/cystic change compared with a matched control group.

Study Design:

Cohort study; Level of evidence, 3.

Methods:

The records of all patients who underwent arthroscopic hip surgery for FAI at a single institution between 2007 and 2013 were reviewed for subchondral edema/cyst on preoperative MRI. Lesions were characterized by grade using an established classification system and were correlated with arthroscopic articular cartilage changes. A matched cohort of patients without evidence of subchondral edema or cyst was identified. Minimum 2-year outcomes were compared using prospectively collected Hip Outcome Score (HOS) activities of daily living and sport subscales as well as the modified Harris Hip Score (mHHS).

Results:

Overall, 104 patients were included. Thirty-six patients (18 men, 18 women) with a mean age of 41 years (range, 19-67 years) had subchondral edema, with or without the presence of cystic acetabular changes, at minimum 2-year follow-up (range, 24-60 months). Two patients who underwent total hip replacement were excluded in the outcome score comparison. Thirty-one of 34 patients (91%) had a grade 4 full-thickness cartilage lesion at the time of diagnostic arthroscopy. The mean mHHS was inferior for all patients with subchondral edema/cystic change (79.9 ± 18.7 vs 86.6 ± 12.5; P = .03), and the HOS was also lower (69.1 ± 27.0 vs 79.5 ± 21.4; P = .02). The overall success rate was 67% for all patients with subchondral edema/cystic change compared with 85% in the control group (P = .04).

Conclusion:

The presence of a subchondral edema with an acetabular cyst on MRI is indicative of a full-thickness cartilage lesion at the time of arthroscopy. These patients have inferior outcomes for arthroscopic treatment of FAI compared with patients with similar age and activity level without MRI subchondral cystic changes.

Keywords

hip femoroacetabular impingement hip labral tear acetabular cyst acetabular edema

Femoroacetabular impingement (FAI) is a structural disorder causing abnormal contact between the acetabulum and the femoral head-neck junction that can cause tears of the acetabular labrum.¹⁸ FAI is an increasingly recognized source of pain and dysfunction in the young adult.^2,8,13,14 In addition to labral tearing, this repetitive contact can also lead to articular cartilage damage with development of arthritis in the young patient, and up to 90% of young patients that develop osteoarthritis of the hip have an underlying structural problem.⁹

The outcome for hip arthroscopy to treat FAI is generally favorable. However, multiple previous studies suggest that patients with coexisting arthritis do poorly with arthroscopy.^1,3,14,26 Phillipon et al²¹ previously found that joint space <2 mm portends a worse prognosis in patients undergoing hip arthroscopy. Furthermore, Byrd and Jones³ demonstrated that the presence of Tönnis grade 3 radiographic changes (severely narrowed joint space) resulted in uniformly poor results, with 7 of 8 patients converted to hip arthroplasty. However, there remains a subset of hips in young patients with relatively maintained joint space (Tönnis grade ≤2) but with subchondral acetabular changes including edema and cysts on magnetic resonance imaging (MRI). Although subchondral cysts and edema suggest degenerative cartilage changes in symptomatic patients, they can be present in asymptomatic hips as well.²³ Overall, the clinical outcome of hip arthroscopy in hips with relatively well-maintained joint space but subchondral bony changes including edema with or without cysts on MRI is unknown.

We designed this study to assess the clinical outcomes of arthroscopic treatment for FAI in hips with subchondral acetabular changes compared with similar hips without any edema or cystic changes. The purpose of this study was to (1) correlate preoperative MRI findings of subchondral acetabular edema with or without cystic change with intraoperative arthroscopy grading of articular cartilage, and (2) to determine whether postoperative outcome was worse for patients with subchondral edema and/or subchondral cyst, compared with a matched control group. We hypothesized that subchondral edema with or without acetabular cysts is associated with advanced intraarticular degenerative changes, and hips with these changes will have inferior clinical outcomes. This may be clinically useful to aid in surgical decision making and may help to identify appropriate candidates for hip arthroscopy.

Methods

Study Design

This investigation was a retrospective review of prospectively collected data including patients who underwent hip arthroscopy for FAI at our institution between 2007 and 2013. A prospectively collected database for hip arthroscopy was used to identify 530 patients who underwent surgery between the listed dates. All patients gave written consent to participate in this study, and the study was approved by our institutional review board. Preoperative MRI was used to identify patients with evidence of subchondral edema with or without subchondral cysts in the acetabulum (subchondral edema/cystic change group). Bone marrow edema lesions were assessed for this group using an established radiographic MRI classification system for subchondral edema and cyst formation.¹⁵ The MRI scans were read by an experienced board-certified orthopaedic surgeon. Specifically for edema, absent lesions were graded as 0, lesions ≤0.5 cm were graded as 1, lesions between 0.5 and 1.5 cm were graded as 2, and lesions ≥1.5 cm were graded as 3. Specifically for subchondral cysts, absent lesions were graded as 0, lesions ≤0.5 cm were graded as 1, and lesions >0.5 cm were graded as 2 (Table 1); these patients were subsequently subdivided into 2 groups: subchondral edema alone and subchondral edema with acetabular cystic changes.

Table 1

Magnetic Resonance Imaging Grading for Features of Hip Arthritis^a

Grade	Bone Marrow Edema Lesion, cm	Subchondral Cyst, cm
0	Absent	Absent
1	≤0.5	≤0.5
2	0.5-1.5	>0.5
3	≥1.5	n/a

Grading as described by Kumar et al.¹⁵ n/a, not applicable.

Inclusion/Exclusion Criteria

The study population included patients with FAI with cam-type and/or pincer lesions who underwent primary arthroscopic treatment by 2 experienced orthopaedic surgeons (A.J.K. and B.A.L.) from 2007 to 2013. Patients were eligible for the study if they were aged >17 years and were diagnosed with either pincer-type FAI, cam-type FAI, or mixed (coexistent pincer and cam types) FAI. The diagnosis of pincer- and cam-type FAI was made based on history, positive impingement signs on examination, and radiographic evidence for pincer-type FAI. Concomitant cartilage treatment was also recorded (chondroplasty, microfracture, or both). In addition, the presence of labral tear/pathology was required on MRI for all patients.

Patients were excluded if they had <2 years of follow-up, previous hip surgery, Tönnis grade 2 osteoarthritis (with <2 mm of joint space narrowing) or any grade 3,²⁸ and/or radiographic evidence of hip dysplasia.^4,29

Thirty-eight patients were initially identified. Two patients were lost to follow-up before their 1-year appointment and were excluded. Two patients in the subchondral edema group underwent total hip replacement and are included in the overall success/failure rate analysis. However, they were excluded in the outcome score comparison and matching process because the last clinical outcome scores reflect their outcome after hip arthroplasty as opposed to hip arthroscopy.

A matching cohort of patients who did not have evidence of acetabular subchondral edema or cysts (control group) was identified by reviewing medical records, operative reports, and radiographic images. The radiographic images were obtained from the medical records and were read by a board-certified orthopaedic surgeon. Both 1.5- and 3.0-T scanners (1.5-T or 3-T Signa HDX 16.0; GE Healthcare) were utilized. A matched cohort of patients without evidence of subchondral edema or cysts was identified, with 2:1 matching based on patient age, surgeon, surgery year within 12 months, surgical procedure (labral debridement vs labral repair), baseline Tegner activity score, and Tönnis grade osteoarthritis changes on preoperative radiographs. All patients in both groups were Tönnis grade 1 or 2 and were matched by Tönnis grade. All Tönnis grade 2 hips had >2 mm of joint space remaining. Selection of cases and controls in a 2:1 ratio increases precision of the results along with the power to detect smaller statistically significant changes. This is especially relevant when there is a concern regarding sufficient significance in stratified or subgroup analyses. Additional benefits include controlling for confounding variables when the sample size is small.¹¹

Surgical Procedure

All patients underwent hip arthroscopy in the supine position. Diagnostic arthroscopy included grading of articular cartilage lesions based on the Outerbridge classification,²⁰ and chondrolabral delamination was classified according to Beck et al.¹ Cam and pincer resection was performed with fluoroscopic guidance. During the time period of inclusion, no routine capsular repairs were performed. To ensure similar surgical technique, patients were matched within the same year of surgery because surgical techniques for FAI evolved rapidly over the entire study time period. Arthroscopic labral debridement (26%) or repair (74%) was performed as dictated by the findings at the time of surgery. All patients were managed with the same postoperative pain regimen and rehabilitation protocol.

Functional and Clinical Assessment

Patients were routinely asked to complete 2 questionnaires at 12 months, 24 months, and 60 months postoperatively to determine clinical outcome. These questionnaires included the modified Hip Harris Score (mHHS) and the Hip Outcome Score (HOS). The mHHS has been used commonly after hip arthroscopy and has high validity.^7,10,22,27 The HOS is shown to be a reliable, valid, and responsive measure of hip function.^15,19 For this study, the HOS subscales for activities of daily living (ADL) and sport were used in the analysis. In addition, patients were asked to rate their current level of function as normal, nearly normal, abnormal, or severely abnormal.

Clinical Success/Failure

Overall success was defined as a hip activity level rating of normal or nearly normal based on the HOS-ADL, which has been shown to be a reliable, valid, and responsive measure of hip function.^15,19 Failure was defined as the combination of conversion to hip replacement or a rating of abnormal or severely abnormal on these patient-reported outcome measures.

Statistical Analysis

Comparisons of patient characteristics between subchondral edema with or without cysts and a matched control group, as well as subgroup analysis (bone marrow edema only vs subchondral cysts and bone marrow edema), were conducted using the Wilcoxon nonparametric signed-rank test for continuous variables (HOS-ADL, HOS-Sport, and mHHS). For categorical variables (success/failure and HOS functional subsets), independent associations were evaluated using the Fisher exact test. All analysis was 2-tailed. P < .05 was considered significant. Analyses were performed using JMP statistical software (version 7; SAS Institute Inc).

Results

Overall, 104 patients were included in this study. Thirty-six patients (18 male, 18 female) with a mean age of 41 years (range, 19-67 years) met inclusion criteria for the subchondral edema/cystic change group (Figure 1). Further subdivision identified 5 patients with subchondral edema alone and 29 patients with subchondral edema as well as cystic change. Sixty-eight patients (21 male, 47 female) with a mean age of 41 years (range, 18-69 years) were included in the matched control group. Patients were followed for a minimum of 2 years postoperatively (range, 24-60 months).

Overall, the presence of subchondral edema/cystic changes was indicative of advanced articular cartilage degeneration, with 31 of 34 patients (91%) demonstrating grade 4 articular cartilage loss of the acetabulum at the time of arthroscopy based on Outerbridge and Beck classifications (Figure 2). Cartilage treatment for this group included chondroplasty in 28 patients (83%) and microfracture in 6 patients (17%). In addition, 100% of patients with subchondral cystic change with area >5 mm had evidence of an adjacent grade 4 full-thickness cartilage lesion at the time of hip arthroscopy (13 patients).

Patients had similar age, activity levels, and Tönnis grades at baseline and had similar surgery and cartilage treatment in the subchondral edema/cystic change group and control groups (Table 2). Mean mHHS results were inferior for the subchondral edema/cystic change group (79.9 ± 18.7) compared with the control group (86.6 ± 12.5; P = .03) (Table 3) and HOSs were significantly lower (69.1 ± 27.0 vs 79.5 ± 21.4; P = .02). Patients rated their hip as subjectively worse in the subchondral edema/cystic change group (Table 4), and the overall success rate was inferior at 67% in subchondral edema/cystic change group compared with 85% in the control group (P = .04) (Table 5). Failure was defined as conversion to hip replacement or a rating of abnormal or severely abnormal.

Table 2

Comparison of Baseline Characteristics Between Patients With Subchondral Edema and Controls^a

Characteristic	All Patients With Subchondral Edema (n = 36)	Matched Controls (n = 68)
Mean age, y	40.4 (19-67)	40.9 (18-69)
Tegner score	3.88 (1-9)	4.04 (1-9)
Sex, female/male, n	18/18	47/21

Values are reported as mean (range) unless indicated otherwise.

Table 3

Comparison of Outcome Scores for Patients With Subchondral Edema Versus Controls^a

Outcome Measure	All Patients With Subchondral Edema	Matched Controls	P Value
mHHS	79.9 ± 18.7	86.6 ± 12.5	.03^b
HOS-ADL	83.3 ± 18.6	88.8 ± 13.2	.06
HOS-Sport	69.1 ± 27.0	79.5 ± 21.4	.02^b

Values are reported as mean ± SD. HOS-ADL, Hip Outcome Score activities of daily living subscale; HOS-Sport, Hip Outcome Score sport subscale; mHHS, modified Harris Hip Score.

Statistically significant difference between groups.

Table 4

Comparison of Subjective Patient-Rated Function Between Patients With Subchondral Edema and Controls^a

HOS Function Rating	Patients With Subchondral Edema (n = 34)	Matched Controls (n = 68)
Normal	9 (26)	22 (32)
Nearly normal	15 (44)	36 (53)
Abnormal	8 (24)	8 (12)
Severely abnormal	2 (6)	2 (3)

Values are reported as n (%). HOS, Hip Outcome Score.

Table 5

Overall Success/Failure Rates in Patients With Subchondral Edema and Controls^a

	All Patients With Subchondral Edema (n = 36)	Matched Controls (n = 68)	P Value
Success	24 (67)	58 (85)	.04
Failure	12 (33)	10 (15)

Values are reported as n (%). Success was defined as a Hip Outcome Score function rating of normal or nearly normal; failure was defined as a rating of abnormal or severely abnormal or as conversion to total hip arthroplasty (n = 2 patients).

In the subgroup analysis, 5 patients were identified with subchondral edema alone (4 hips with grade 4 change, 1 hip with grade 3 change), and 29 were identified with subchondral edema as well as presence of cystic change (27 with grade 4 change, 2 with grade 3 change). For the group with subchondral edema with cystic change, the mean mHHS was inferior (70.6 ± 18.7 vs 86.6 ± 12.5; P < .01), HOS showed lower scores (64.3 ± 23.9 vs 79.5 ± 21.4; P < .01), and the overall success rate was 66% compared with 85% in the matched control group. For the group with subchondral edema only, mean mHHS (82.4 ± 25.0 vs 86.6 ± 12.5; P = 0.8) and HOS results (72.4 ± 38.1 vs 79.5 ± 21.4; P = .9) were not inferior, and the overall success rate for this group was 80%.

Discussion

FAI leads to labral tears, articular cartilage wear, and subsequent arthritis. Arthroscopic treatment for FAI has demonstrated satisfactory outcomes; however, the success of any treatment for FAI is limited by the coexistence and severity of osteoarthritis. Previously, the contribution of acetabular bony findings on MRI to clinical outcome after hip arthroscopy was unknown. Therefore, we designed this study to determine the significance of subchondral edema and cystic change in the setting of younger patients with FAI. We found that acetabular cystic changes were predictive of advanced cartilage changes at the time of arthroscopy. In addition, the presence of these bony changes was associated with inferior outcome after arthroscopic FAI surgery compared with a control group.

A significant proportion of patients undergoing hip arthroscopy have degenerative changes of the joint. Unfortunately, even thorough assessment of the preoperative radiographs and MRI cannot always predict the status of the articular cartilage at the time of surgery. For instance, 88% of patients who did not have significant osteoarthritis on preoperative radiographs were found to have intraoperative acetabular articular cartilage lesions at the time of hip arthroscopy.¹⁷ For MRI assessment of articular cartilage, the previous correlation with intraoperative arthroscopy findings has been disappointing, with a sensitivity of only 47%.¹² In this study, we found that edema and subchondral cystic change, especially over 5 mm, were indicative of advanced cartilage changes in the hip joint at the time of arthroscopy. This is valuable information to assist counseling patients preoperatively if these changes are observed on their MRI.

In the knee, there has been a clear relationship established between subchondral edema and cystic changes and the presence of full-thickness cartilage lesions. Roemer et al²⁵ reported on a longitudinal multicenter study of knee arthritis, and they found that new bone marrow lesions demonstrated a high risk for subsequent cartilage loss over 30 months. However, the cause of subchondral cystic lesions is still unknown. One theory suggests that increased intraarticular pressure leads to intrusion of the synovial fluid into the subchondral bone through cartilage loss or fissured cartilage.¹⁶ Another theory is that subchondral cysts are the sequelae of traumatic bone necrosis after impact of 2 articulating cartilage surfaces.²⁴ This second theory would seem very plausible in the setting of FAI, where there is repetitive abnormal contact. In addition, a longitudinal knee MRI study demonstrated that bone marrow lesions strongly predicted the formation of subchondral cysts in the same subregion of bone, even after adjusting for full-thickness cartilage loss and fissures, supporting the bone contusion theory of subchondral cyst formation.⁵ This would suggest that the bone marrow edema lesions in the hip may be precursors and progress to cystic changes, but further work needs to be performed in this area.

In the current study, patients in the group with subchondral edema with or without cysts had worse outcomes compared with the control group with similar age and activity level. This is likely because of the associated arthritic changes and supports previous findings of the negative influence of cartilage degeneration on improvement after hip arthroscopy. Farjo et al⁸ reported good or excellent results in 71% of patients without arthritis after hip arthroscopy, but only 21% good to excellent results in patients with arthritis. After hip arthroscopy, Larson et al¹⁷ found a 52% failure rate, as defined by conversion to hip arthroplasty or mHHS <70, in the arthritic group compared with 12% in the nonarthritic group. However, questions remain regarding how much arthritis is too much for successful results of hip arthroscopy. A recent systematic review by Domb et al⁶ included 13 studies and showed that patient-reported outcome scores, similar to the ones used in this study, are lower in the arthritic population compared with individuals with nonarthritic hips. Nevertheless, as Egerton et al⁷ pointed out, many patients with cartilage degeneration still improve after hip arthroscopy. It is important to keep in mind that many patients with arthritis will have lower baseline patient-reported outcomes, and although the postoperative scores may also be lower, the change in score reflects a potentially clinically beneficial result to patients.⁷ In this study, the group with subchondral edema with or without cysts did worse, but two-thirds of patients still benefited from surgery at short-term follow-up. Therefore, hip arthroscopy may still be warranted for younger patients who are not arthroplasty candidates.

This study used a well-matched control group and prospectively collected patient-reported outcomes. However, limitations include the relatively small cohort and retrospective analysis. The small cohort makes it more difficult to completely assess risk factors for failure. Furthermore, stratification of the group into subgroups with subchondral edema alone and subchondral edema with cystic change yielded only 5 patients with subchondral edema alone. When comparing this subgroup with the control group, no significant difference was noted for final functional outcomes; however, this analysis is likely to be underpowered. In addition, the comparison of postoperative scores does not reflect the change from baseline scores in these patients. It is possible that patients in both groups had a similar improvement in outcome scores. This study emphasizes the importance of recognizing subchondral edema and cystic change on MRI when considering hip arthroscopy. It provides valuable information for surgical decision making to counsel patients that subchondral edema represents advanced degenerative changes despite mild radiographic appearance, and that chondral degeneration portends a less favorable outcome after hip arthroscopy.

Conclusion

The presence of a subchondral acetabular cyst on MRI is indicative of a full-thickness cartilage lesion at the time of arthroscopy. These patients have inferior outcomes for arthroscopic treatment of FAI compared with patients with similar age and activity level without MRI subchondral changes. Therefore, we recommend caution when considering hip arthroscopy in these patients.

Footnotes

Presented at the 41st annual meeting of the AOSSM, Orlando, Florida, July 2015.

One or more of the authors has declared the following potential conflict of interest or source of funding: A.J.K. is a paid consultant for Arthrex Inc and receives research support from the Arthritis Foundation and Histogenics. B.A.L. receives royalties from Arthrex Inc and VOT Solutions, is a paid consultant for Arthrex Inc, and receives research support from Arthrex Inc, Biomet, and Stryker.

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