Abstract
The treatment of adult borderline hip dysplasia remains challenging and continues to be a subject of controversy. The preferred treatment for hip instability and acetabular undercoverage is periacetabular osteotomy. However, patients with painful hips and associated femoroacetebular impingement, microinstability, or no instability may benefit from arthroscopic surgery. Short-term studies have reported favorable clinical outcomes. Traditionally, the lateral center-edge angle was used to determine hip dysplasia. More recently the femoro-epiphyseal acetabular roof (FEAR) index was introduced as a measure for borderline dysplasia. In general, a FEAR index of less than 5° indicates hip instability. When using a FEAR index of more than 2° as a cut-off for hip instability and borderline dysplasia, arthroscopic hip surgery can achieve very similar clinical outcomes to patients with a FEAR index of less than 2°. However low and unequal sample sizes have potentially resulted in both type I and II errors, reducing internal study validity. Although this may be a step in the right direction, further high-quality studies are required to understand patients’ characteristics on diagnosis, prognosis, outcomes of surgical interventions, and long-term disease progression for adult borderline hip dysplasia.
Nearly 20 years ago, Byrd and Jones performed hip arthroscopy in 19 cases with hip dysplasia and 32 cases of borderline dysplasia.
1
They reported an increase in the modified Harris Hip Score (mHHS) by 27 points to 77 points at the 2-year follow-up and noted that 79% of all patients improved at least by 10 points.1
However, if we apply the minimal clinically important difference (MCID) or patient acceptable symptomatic state (PASS) to the mean values reported by Byrd and Jones, they do not meet MCID nor PASS. The MCID for mHHS has been reported to be 20 and the PASS to be 84.2
However, here is a disclaimer: strictly speaking, both MCID and PASS cannot be used for mean value interpretation, but at least it may give us an “idea.” However, favorable outcomes have been reported later by various studies suggesting that hip arthroscopy may have a role in patients with FAI and borderline dysplasia.- Chahal J.
- Van Thiel G.S.
- Mather R.C.
- Lee S.
- Salata M.J.
- Nho S.J.
The minimal clinical important difference (MCID) and patient acceptable symptomatic state (PASS) for the modified Harris Hip Score and Hip Outcome Score among patients undergoing surgical treatment for femoroacetabular impingement.
Orthop J Sports Med. 2014; 2 (2325967114S00105 (2 suppl))
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, 5
, 6
Traditionally, the lateral center-edge angle has been used to determine hip dysplasia, and a lateral center-edge angle between 15 and 25° suggests the presence of borderline dysplasia.
5
,6
Recently, Wyatt et al.7
have introduced the FEAR (femoro-epiphyseal acetabular roof) index as a new measure to determine borderline dysplasia. The authors have shown that a FEAR index of less than 5° has a 79% probability of correctly assigning hips as stable with a specificity and sensitivity around 80%.7
Later, Smith et al.8
have shown that a FEAR index greater than 1.3° were borderline with a receiver operator characteristic area under the curve of 0.86. In their series of 220 patients, 90% of patients with values above 1.3° were dysplastic.8
Wong, Newhouse, Wichman, Bessa, Willialms, and Nho
9
have now reported clinical outcomes in patients with borderline dysplasia undergoing hip arthroscopy for femoroacetabular impingement. In their study titled “Patients With a High Femoroepiphyseal Roof With Concomitant Borderline Hip Dysplasia and Femoroacetabular Impingement Syndrome Do Not Demonstrate Inferior Outcomes Following Arthroscopic Hip Surgery,” the authors have compared 19 patients with a FEAR index greater than 2° with 121 patients with a FEAR index of less than 2°.9
Postoperative mHHS was 80 in the stable and 79 in the unstable group; similar scores were obtained for Hip Outcome Score (HOS)-Activities of Daily Living and HOS-Sport Scale.9
MCID was achieved in 73% to 79% in the stable group and in 71% to 91% in the unstable group; PASS ranged from 59% to 65% in the stable and 55% to 63% in the unstable group. 9
The between-group differences were not statistically different, suggesting that patients with borderline dysplasia have very similar outcomes when compared with patients with a normal hip. Good news.But hold on—there is a potential problem with this conclusion. The sample size for both groups was substantially different; only 19 patients were included in the borderline dysplasia group and 121 patients were enrolled to the normal group. Unequal sample sizes can affect the robustness of equal variance assumptions when performing statistical analysis and increase the possibility of a type I error.
10
As such, the lack of differences may simply be caused by a type I error. It is not clear whether the authors have adjusted their analysis by using the Welch t-test or assuming unequal variances when performing their independent samples t-tests. What about a type II error? Using a 6:1 ratio accounting for unequal samples sizes and calculating the sample size required (based on the MCID for HOS-Sport Scale), 29 patients for the instability group and 174 patients in the stable group would have been required for an adequately powered study. Type II error? Most likely. And, the authors have admitted this in their results section. This means that the authors’ conclusions need to be interpreted very carefully.However, the results of Wong et al. study are similar to other studies. Tang and Dienst
5
have performed a systematic review in 2020 and included 5 studies, 2 case series, and 3 studies with a control group. They reported that improved outcomes were observed at the 2-year follow-up period in 4 of the 5 studies with no significant differences in postoperative outcomes between normal and borderline hip dysplasia patients.5
Unfortunately, the included studies also suffered from relatively low sample sizes. Kuroda et al.11
have also performed a systematic review and included 28 studies with a total of 1502 hips. Similar to Tang and Dienst,5
the authors reported an improvement in patient-reported outcomes for capsular plication with a low complication rate in the shorter term.11
Older age, excessive femoral anteversion, and anterior acetabular undercoverage were risk factors for poorer outcomes.11
However, the majority of studies had short-term follow-up, and longer-term follow-up periods can easily ruin great results.Longer term follow-up data were reported by Domb et al.
12
The group has followed 24 patients who underwent hip arthroscopy with labral preservation and capsular plication for 5 years and did not notice a drop off in patient-reported outcome scores.12
Encouraging, and we hope that Wong et al. will report their longer follow-up data. Perhaps arthroscopic treatment of femoroacetabular impingement and improving joint stability by addressing labral tears and capsular laxity may become a feasible alternative to periacetabular osteotomies (PAO) in the future. Obviously, arthroscopy cannot address bony defects and osseous hip instability and PAO will always have a role in addressing these deformities.13
That said, it seems that we are heading in the right direction. Similar to arthroscopic shoulder stabilization, which was initially met with resistance and has now superseded open Bankart repair, hip arthroscopy may eventually replace PAO for soft-tissue–related hip instability and femoroacetabular impingement in patients with borderline hip dysplasia. Currently, high-quality level I evidence studies are rare but are required to understand patients’ characteristics on diagnosis, prognosis, outcomes of surgical interventions, and long-term disease progression for adult borderline hip dysplasia.
Supplementary Data
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References
- Hip arthroscopy in the presence of dysplasia.Arthroscopy. 2003; 19: 1055-1060
- The minimal clinical important difference (MCID) and patient acceptable symptomatic state (PASS) for the modified Harris Hip Score and Hip Outcome Score among patients undergoing surgical treatment for femoroacetabular impingement.Orthop J Sports Med. 2014; 2 (2325967114S00105 (2 suppl))
- Does hip arthroscopy have a role in the treatment of developmental hip dysplasia?.J Arthroplasty. 2017; 32: S28-S31
- The role of arthroscopy in the dysplastic hip- a systematic Review of the intra-articular findings, and the outcomes utilizing hip arthroscopic surgery.J Hip Preserv Surg. 2016; 9: 171-180
- Surgical outcomes in the treatment of concomitant mild acetabular dysplasia and femoroacetabular impingement: A systematic review.Arthroscopy. 2020; 36: 1176-1184
- Do patients with borderline dysplasia have inferior outcomes after hip arthroscopy for femoroacetabular impingement when compared to patients with normal coverage?.Am J Sports Med. 2017; 45: 2116-2124
- The femoro-epiphyseal roof (FEAR) index: A new measurement associated with instability in borderline hip dysplasia.Clin Orthop Relat Res. 2017; 475: 861-869
- Can the femoro-epiphyseal acetabular roof angle (FEAR) index be used to distinguish dysplasia from impingement?.Clin Orthop Relat Res. 2021; 479: 962-971
- Patients with a high femoroepiphyseal roof with concomitant borderline hip dysplasia and femoroacetabular impingement syndrome do not demonstrate inferior outcomes following arthroscopic hip surgery.Arthroscopy. 2022; 38: 1509-1515
- Impact of sample size and variability on the power and type I errors of equivalence tests: A simulation study.Pract Assess Res Eval. 2014; 19: 11
- Hip arthroscopy and borderline developmental dysplasia of the hip: A systematic review.Arthroscopy. 2020; 36: 2550-2567
- Hip arthroscopic surgery with labral preservation and capsular plication in patients with borderline hip dysplasia: Minimum 5-year patient-reported outcomes.Am J Sports Med. 2018; 46: 305-313
- Editorial commentary: Arthroscopy for borderline developmental dysplasia of the hip: Selection determines the outcomes.Arthroscopy. 2020; 36: 2568-2571
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Footnotes
The author reports the following potential conflicts of interest or sources of funding: personal fees from the AANA, outside the submitted work. Full ICMJE author disclosure forms are available for this article online, as supplementary material.
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