Does using constrained acetabular component really limit hip range of motion?

Aim: In surgical treatment of instability, constrained acetabular inserts are frequently used in hip arthroplasty. However the reasons why surgeons avoid constrained acetabular components are the concern of an increased rate of loosening possibly due to impingement and the concern of decreased range of motion. This study aims to investigate the influence of constrained acetabular insert usage on hip range of motions and functional results. Material and Methods: Twenty-eight patients who needed revision hip arthroplasty were included. Patients were divided into two groups according to acetabular insert used in surgery (constrained and non-constrained). Mean follow-up period was 61±7 months (range, 50-74) in constrained group and 59±7 (range, 50-72) in non-constrained group. Hip range of motion and harris hip scores were recorded pre-operatively and at final follow-up. Results: The final avarage flexion, abduction, adduction, external rotation and internal rotation was respectively 78°±15°, 43°±4°, 28°±3°, 30°±7°,19°±8° in constrained group (n=15) and 75°±14°, 40°±6°, 26°±5°, 30°±12°, 17°±6° in non-constrained group (n=13). The difference between groups was not statistically significant. Harris hip score increased in both groups and there was no significant difference between groups (p=0.730). Conclusion: Findings of this mid term study showed that hip range of motions and functional results in patients with constrained acetabular inserts are not inferior than the patients with non-constrained inserts.


Introduction
Total hip replacement is one of the most satisfying procedures in orthopaedics [1]. On the other hand, in the course of time, hip prothesis may fail because of several reasons. Today hip revision arthroplasty is more needed since more total hip arthroplasty procedures are being performed especially on younger patients [2].
One of the most common problems after revision hip surgery is instability [3]. The reported incidence varies up to 35% after revision arthroplasty [4]. Although instability can successfully be treated conservatively, in many instances surgery may be required, especially in recurrent instabilities [5]. Surgical options include proper readjustment of acetabular and femoral component orientation, exchange of modular components such as femoral head and acetabular liner, usage of larger femoral head, soft tissue reinforcement, advencement of greater trochanter and using a dual mobility implant or a constrained component [6][7][8].
Constrained acetabular component prevents instability by holding femoral head captive within the socket [8,9]. It is an option for patients with recurrent dislocation, intraoperative instability, instability of unknown etiology, abductor deficiency, neuromuscular and cognitive disorders [10,11].
There have been concerns about constrained components if there is any decreasing effect on hip range of motion (ROM) [10][11][12][13][14]. However, there is no study comparing in vivo hip ROM of constrained acetabular components with those of nonconstrained ones. Thus this study was designed to compare ROM and functional score of the patients operated using constrained with non-constrained components.

Material and Methods
Between November 2013 and November 2015, patients who was in need of revision hip arthroplasty and admitted to Ankara Numune Training and Research Hospital were included in this prospective non-randomized controlled study. This

Statistical analysis
Statistical anaylsis was performed using PASW Statistics for Windows (version 18, USA). Normal distribution of the parameters in each group was screened with Shapiro-Wilk test. Mann Whitney U test was used to compare groups. A p-value <0.05 was considered statistically significant.

Results
The baseline characteristics were comparable in the two groups (Table 1). Constrained group had higher hip ROM (flexion, abduction, adduction, external rotation (in extension) and internal rotation (in extension)) when compared with non-constrained group, however the difference between groups was not significant ( Table 2).
Harris hip scores increased in both groups when compared with pre-operative values but there was no significant difference between groups (p=0.730). In the constrained group harris hip score improved from a mean of 41.80 ± 17.82 None of our patients had pain at the end of the ROM, none of them suffered about a sense of impingement, blockade or elastical fixation. Also, none of our patients had radiolucent line around the components occupying more than 50% of the prosthesis-bone interface on any radiograph or none of our patients were with progressive radiolucent line suggesting loosening or implant wear.
At follow-up period no patient underwent re-revision in neither constrained nor non-constrained group. No dislocation, infection or loosening of the components occured.

Discussion
In surgical treatment of instability, constrained acetabular components are frequently used in revision hip arthroplasty. The reasons why surgeons avoid constrained acetabular components are the concern of an increased rate of loosening possibly due to impingement and the concern of decreased range of motion [10][11][12][13][14]. But although the use of constrained acetabular inserts rapidly increased in recent years, almost there is no study evaluating ROM of constrained devicesin vivo.
The aim of our study was to compare constrained acetabular components with non-constrained ones in terms of hip ROM and functional outcomes.
Theoretically, the freedom constrained acetabular inserts provide 110° ROM with a standart 36 mm femoral head which is the only available femoral head option, whereas, neutral non-constrained acetabular inserts give 136° ROM with 36 mm femoral head [17,18]. However actually fibrous adhesions can occur in most patients and one cannot use maximum ROM allowed by liner. Our results has shown that constrained acetabular components doesn't have lesser range of motions when compared with standard non-constrained components in vivo.
There are a lot of clinical studies related with survival of constrained devices. Studies reported that constrained devices have good short to medium term and poor long term survival rates [19][20][21][22][23][24][25]. Poor long term results are attributed ROM limitation of constrained acetabular components in literature [10][11][12][13][14]. But these results may be a result of implant selection bias, because constrained devices may have utilized in more difficult revision cases [23]. In our clinical study, ROMs of constrained patient group was not lesser than non-constrained group.
After revision hip arthroplasty, complications may occur for a variety of reasons. Springer et al. reported complication rate requiring re-revision was 13% (141 of 1100 patients) with a mean follow-up period of 6 years after revision surgery. These   [4]. In our study, no major complication occured at 28 patients with a mean follow-up period of 60 months and there was no significant difference between groups in terms of complications.
To our knowledge, this is the first study comparing in vivo ROMs of constrained acetabular inserts with non-constrained ones. In this study there are two prominent limitations. First, the patient cohort was small. Second, though follow-up period was sufficient for analysing ROM, it was short for analysing long term survival rates.

Conclusion
Findings of the current study indicated that hip ROMs and functional results in patients with constrained acetabular inserts are not inferior than the patients with non-constrained inserts. Further studies with larger series are wanted to support the results of this work.