Does Surgical Approach Influence the Natural History of the Unstable Total Hip Replacement?

Open AccessPublished:December 16, 2021DOI:https://doi.org/10.1016/j.arth.2021.12.012

      Abstract

      Background

      Dislocation following total hip arthroplasty (THA) is a significant complication that occurs in 0.3%-10% of cases with 13%-42% of patients requiring revision surgery. The literature has primarily focused on the dislocation risk associated with different surgical approaches. However, little is known about the natural history of the dislocated hip and whether surgical approach of the index THA is associated with further instability and revision surgery.

      Methods

      This is a retrospective, single-center, multi-surgeon consecutive case series of all patients who experienced THA dislocation from 2002 to 2020. Patients were excluded if the initial dislocation was secondary to infection or fracture. The natural history of the cohort as per approach was determined. Outcome measurements of interest were the number of dislocations; the treatment surrounding each dislocation; the necessity and type of revision; and the complications encountered.

      Results

      Of the 75 patients, 58 (77%) dislocated within 6 months following primary THA. The anterior group had greater odds of dislocation within 2 weeks post-THA compared to the lateral and posterior groups (P = .04). The mean number of dislocations per patient was significantly lower in the anterior (1.5 ± 0.7) compared to the lateral (2.4 ± 1.2) and posterior (2.1 ± 1.0) groups (P = .02). Revision surgery was needed in 30% (6/20) of patients in the anterior, 69% (25/36) of the posterior, and 68% (13/19) of the lateral groups (P = .01).

      Conclusion

      This study illustrates that while primary THA dislocations happen earlier with the anterior approach, they are typically less complicated and have a lower risk of recurrent instability and revision surgery.

      Keywords

      Dislocation of a primary total hip arthroplasty (pTHA) remains a significant clinical challenge [
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      Treatment of recurrent dislocation after total hip arthroplasty using advanced imaging and three-dimensional modeling techniques: a case series.
      ]. With the expected increase in THAs performed annually [
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      Posterior hip precautions do not impact early recovery in primary total hip arthroplasty: a multicenter, randomized, controlled study.
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      ,
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      • Oppermann J.
      • Bruggermann G.-P.
      • Eysel P.
      Dislocation following total hip replacement.
      ], the burden of complications encountered after the index surgery is also expected to rise [
      • Bozic K.J.
      • Kurtz S.M.
      • Lau E.
      • Ong K.
      • Vail D.T.P.
      • Berry D.J.
      The epidemiology of revision total hip arthroplasty in the United States.
      ,
      Canadian Institute for Health Information
      Hip and Knee Replacements in Canada, 2017-2018.
      ,
      • Charissoux J.L.
      • Asloum Y.
      • Marcheix P.S.
      Surgical management of recurrent dislocation after total hip arthroplasty.
      ,
      • Padgett D.
      • Warashina H.
      The unstable total hip replacement.
      ,
      • Van Der Merwe J.M.
      Comprehensive review of current constraining devices in total hip arthroplasty.
      ,
      • Ulrich S.D.
      • Seyler T.M.
      • Bennett D.
      • Delanois R.E.
      • Saleh K.J.
      • Thongtrangan I.
      • et al.
      Total hip arthroplasties: what are the reasons for revision?.
      ,
      • Rogers M.
      • Blom A.W.
      • Barnett A.
      • Karantana A.
      • Bannister G.C.
      Revision for recurrent dislocation of total hip replacement.
      ]. The incidence of dislocation is variable and ranges from 0.2% to 10% [
      • Charissoux J.L.
      • Asloum Y.
      • Marcheix P.S.
      Surgical management of recurrent dislocation after total hip arthroplasty.
      ,
      • Berry D.J.
      • Von Knoch M.
      • Schleck C.D.
      • Harmsen W.S.
      Effect of femoral head diameter and operative approach on risk of dislocation after primary total hip arthroplasty.
      ,
      • Parvizi J.
      • Picinic E.
      • Sharkey P.
      Revision total hip arthroplasty for instability: surgical techniques and principles.
      ]. THA dislocations can be treated with nonoperative measures such as a closed reduction; however, revision surgery is sometimes needed to achieve stability (Fig. 1) [
      • Bozic K.J.
      • Kurtz S.M.
      • Lau E.
      • Ong K.
      • Vail D.T.P.
      • Berry D.J.
      The epidemiology of revision total hip arthroplasty in the United States.
      ,
      Canadian Institute for Health Information
      Hip and Knee Replacements in Canada, 2017-2018.
      ,
      • Charissoux J.L.
      • Asloum Y.
      • Marcheix P.S.
      Surgical management of recurrent dislocation after total hip arthroplasty.
      ,
      • Padgett D.
      • Warashina H.
      The unstable total hip replacement.
      ,
      • Van Der Merwe J.M.
      Comprehensive review of current constraining devices in total hip arthroplasty.
      ,
      • Ulrich S.D.
      • Seyler T.M.
      • Bennett D.
      • Delanois R.E.
      • Saleh K.J.
      • Thongtrangan I.
      • et al.
      Total hip arthroplasties: what are the reasons for revision?.
      ,
      • Rogers M.
      • Blom A.W.
      • Barnett A.
      • Karantana A.
      • Bannister G.C.
      Revision for recurrent dislocation of total hip replacement.
      ]. Recurrent dislocation (2 or more) occurs between 10.3% and 60% of patients after a first dislocation [
      • Charissoux J.L.
      • Asloum Y.
      • Marcheix P.S.
      Surgical management of recurrent dislocation after total hip arthroplasty.
      ,
      • Conroy J.L.
      • Whitehouse S.L.
      • Graves S.E.
      • Pratt N.L.
      • Ryan P.
      • Crawford R.W.
      Risk factors for revision for early dislocation in total hip arthroplasty.
      ,
      • Kotwal R.S.
      • Ganapathi M.
      • John A.
      • Maheson M.
      • Jones S.A.
      Outcome of treatment for dislocation after primary total hip replacement.
      ]. Instability/dislocation is the most common indication for revision surgery (22.5%), followed by mechanical loosening and infection [
      • Bozic K.J.
      • Kurtz S.M.
      • Lau E.
      • Ong K.
      • Vail D.T.P.
      • Berry D.J.
      The epidemiology of revision total hip arthroplasty in the United States.
      ].
      Figure thumbnail gr1
      Fig. 1Radiographic representation of a total hip arthroplasty with (A and B) posterior dislocation and (C and D) revision total hip arthroplasty with a dual mobility component used for recurrent instability.
      Although there is abundant literature on the incidence, risk factors, and management of THA dislocations, information on the natural history following dislocation is sparse. Kotwal et al reported that 60.4% of hips dislocated more than once and the ones with recurrent instability had inferior patient-reported outcomes. Chandler et al [
      • Chandler R.W.
      • Dorr L.D.
      • Perry J.
      The functional cost of dislocation following total hip arthroplasty.
      ] found that patients who had dislocated on several occasions walked significantly slower and had a reduced single limb support time. To date, little information is available to what effect, if any, surgical approaches to the hip may have on the natural history of THA instability.
      The purpose of this study is to describe the natural history of THA dislocations. In doing so, we set out to determine whether the surgical approach used during the primary procedure (anterior, lateral, or posterior) was associated with increased odds of having recurrent dislocations and requiring subsequent revision surgery to achieve stability.

      Materials and Methods

       Study Design

      This study is a retrospective, single-center, multi-surgeon, consecutive case series of all patients who experienced hip dislocations following pTHA performed at a large, academic, tertiary care center. After Institutional Research Ethics Board approval, a comprehensive list of all dislocations encountered after hip replacement surgery between 2002 and 2020 was retrieved from the institutional database. Exclusion criteria included the following: no true dislocations, bipolar hemiarthroplasty, hip resurfacing arthroplasty (n = 18), missing data on dislocations or approach (n = 8), dislocations secondary to fracture (n = 1; anterior approach [AA]) or infection (n = 1; lateral approach), and dislocations happening more than 5 years after primary surgery (n = 10) since these are often related to polyethylene wear [
      • Charissoux J.L.
      • Asloum Y.
      • Marcheix P.S.
      Surgical management of recurrent dislocation after total hip arthroplasty.
      ].

       Institutional Practice

      The anterior (Hueter), direct lateral (modified direct lateral/Bourne modification) [
      • Mulliken B.
      • Rorabeck C.
      • Bourne R.
      • Nayak N.
      A modified direct lateral approach in total hip arthroplasty: a comprehensive review.
      ], and posterior (with enhanced posterior soft tissue repair) [
      • Pellicci P.M.
      • Bostrom M.
      • Poss R.
      Posterior approach to total hip replacement using enhanced posterior soft tissue repair.
      ] approaches were all performed by high-volume, fellowship-trained orthopedic surgeons. Our institution started performing the AA in 2006. The annual rate of AA THA increased from 1.5% to 53.2% from 2006 to 2017 [
      • Gofton W.T.
      • Ibrahim M.M.
      • Kreviazuk C.J.
      • Kim P.R.
      • Feibel R.J.
      • Beaulé P.E.
      Ten-year experience with the anterior approach to total hip arthroplasty at a tertiary care center.
      ]. During the study period (18 years), 24% (n = 1667) of the pTHAs were performed with the posterior approach, 41% (n = 2779) with the lateral approach, and 35% (n = 2383) with the AA. Intraoperative anteroposterior pelvic radiographs were performed during surgeries to check component placement and size, leg length, and offset restoration by half the surgeon during the study period, reflecting individual surgical preference.
      All patients received a standard course of infection and deep venous thrombosis prophylaxis. All patients were subject to 6 weeks of formal hip precautions specific to their surgical approach. A standard 6 weeks of rehabilitation with physiotherapy and daily home exercises was part of the postoperative course. The patients were followed up in clinic at regular intervals (2-, 6-, 12-, 26-, and 52- weeks) with clinical and radiographic assessments. Annual clinical and radiographic follow-ups were performed thereafter.

       Cohort

      Out of 6832 THAs performed, 87 were identified to have dislocated at least once (estimated incidence 1.2%). Twelve were excluded for the aforementioned reasons and 75 THAs were studied further. Of these, 20 THAs previously had an AA (estimated incidence 1.3%), 19 previously had a lateral approach (estimated incidence 1.0%), and 36 previously had a posterior approach (estimated incidence 3.3%) (Fig. 2).
      Figure thumbnail gr2
      Fig. 2Consort diagram. pTHA, primary total hip arthroplasty.
      Surgical and patient characteristics are provided in Table 1. Out of these 75 THAs, 55 were replaced for osteoarthritis, 11 for avascular necrosis, 5 for failed fracture fixation, and 3 for inflammatory arthritis. There were 27 males (36%) and 48 females (64%), with a mean age of 64.0 years (range 19-91, standard deviation 15.0). The most common femoral head diameter was 32 mm. All pTHAs in the cohort were cementless implants. There was a significantly higher use of high offset stems in the posterior group compared to the anterior group. There were no significant differences in the number of patients with dementia, neuromuscular disease, alcoholism, and spine disease/surgery between the groups.
      Table 1Demographics of the Different Study Groups.
      CharacteristicsTotalAnteriorLateralPosteriorP-Value
      Number of hips75201936
      Mean age (y)64 ± 1553 ± 17
      Significant difference, younger patient cohort in the anterior approach group, difference of side of surgery, higher ratio of high offset stems in the posterior compared to the anterior group.
      69 ± 11
      Significant difference, younger patient cohort in the anterior approach group, difference of side of surgery, higher ratio of high offset stems in the posterior compared to the anterior group.
      67 ± 12
      Significant difference, younger patient cohort in the anterior approach group, difference of side of surgery, higher ratio of high offset stems in the posterior compared to the anterior group.
      .001
      Significant difference, younger patient cohort in the anterior approach group, difference of side of surgery, higher ratio of high offset stems in the posterior compared to the anterior group.
      Gender (male/female)(27/48)(8/12)(5/14)(14/22).59
      Side (right/left)(42/33)(2/18)
      Significant difference, younger patient cohort in the anterior approach group, difference of side of surgery, higher ratio of high offset stems in the posterior compared to the anterior group.
      (34/2)
      Significant difference, younger patient cohort in the anterior approach group, difference of side of surgery, higher ratio of high offset stems in the posterior compared to the anterior group.
      (6/13)
      Significant difference, younger patient cohort in the anterior approach group, difference of side of surgery, higher ratio of high offset stems in the posterior compared to the anterior group.
      .001
      Significant difference, younger patient cohort in the anterior approach group, difference of side of surgery, higher ratio of high offset stems in the posterior compared to the anterior group.
      Head diameter (mm)
       222002.001
      Significant difference, younger patient cohort in the anterior approach group, difference of side of surgery, higher ratio of high offset stems in the posterior compared to the anterior group.
       28131102
       323111713
       36298219
      Stem offset (SO/HO)65/1020/0
      Significant difference, younger patient cohort in the anterior approach group, difference of side of surgery, higher ratio of high offset stems in the posterior compared to the anterior group.
      27/9
      Significant difference, younger patient cohort in the anterior approach group, difference of side of surgery, higher ratio of high offset stems in the posterior compared to the anterior group.
      18/1
      Significant difference, younger patient cohort in the anterior approach group, difference of side of surgery, higher ratio of high offset stems in the posterior compared to the anterior group.
      .015
      Significant difference, younger patient cohort in the anterior approach group, difference of side of surgery, higher ratio of high offset stems in the posterior compared to the anterior group.
      Dementia6/751/202/193/36.55
      Neuromuscular disease5/751/201/193/36
      Alcoholism3/750/200/193/36
      Spine pathology/surgery3/752/201/191/36
      Size 26-mm and 28-mm heads were grouped together.
      SO, standard offset; HO, high offset.
      a Significant difference, younger patient cohort in the anterior approach group, difference of side of surgery, higher ratio of high offset stems in the posterior compared to the anterior group.

       Outcome Measures of Interest

      The direction of instability was determined with radiographic and chart review. The type of revision, timing, and rationale of treatment was acquired from a review of operative records. A chart review and a radiographic review were used to determine the timing of dislocation and nonoperative treatment. The outcome measures of interest were related to the natural history following prosthetic hip dislocation and were obtained via a comprehensive chart review. The outcomes included the following: the timing of dislocations, the number of dislocations, the treatment surrounding each dislocation, the necessity for revision and re-revision surgery, and the type of revision surgery. Minor revision was defined as surgery requiring only an exchange of the modular components. A major revision was defined as any revision surgery requiring removal of nonmodular components.

       Analysis Performed

      Data are reported in accordance with STROBE guidelines. The timing of dislocations, number of dislocations, and revisions were described in order to determine the natural history of THA dislocations between surgical approaches. Continuous variables were described using the mean and standard deviation and categorical variables were presented with total count and percentages. Analysis was performed among the anterior, lateral, and posterior approach groups (3 group analysis). Groups were compared between each other (2 group analysis) to determine specific differences and significance between 2 groups. The data were further analyzed by performing a subgroup analysis looking at early dislocations (<6 months after pTHA) and late dislocations (>6 months after pTHA) [
      • Charissoux J.L.
      • Asloum Y.
      • Marcheix P.S.
      Surgical management of recurrent dislocation after total hip arthroplasty.
      ,
      • Goel A.
      • Lau E.C.
      • Ong K.L.
      • Berry D.J.
      • Malkani A.L.
      Dislocation rates following primary total hip arthroplasty have plateaued in the Medicare population.
      ]. Further subgroup analysis was performed looking at 3 specific time intervals post-THA (<2 weeks, >6 weeks, and >6 months). The Pearson chi-squared and Fisher’s exact tests were used to test for differences between categorical variables and the Mann-Whitney U-test was used for continuous variables. When comparing multiple continuous variables, Kruskal-Wallis test (one-way analysis of variance on ranks) was used with a post hoc Bonferroni correction. Significance was set at P < .05. All data were determined to be nonparametric based on the Shapiro-Wilk test. All analyses were performed using IBM SPSS (IBM Corp, Armonk, NY) software for Mac 9 (version 27).

      Results

       Total Number of Dislocations

      There were a total of 152 episodes of dislocations in 75 hips (Table 2). The anterior group had a significantly lower mean number of dislocations per patient with 1.5 ± 0.7 dislocations compared to the lateral (2.4 ± 1.2; P = .03) and the posterior (2.1 ± 1.0; P = .02) groups (Fig. 3, Fig. 4).
      Table 2Total Number of Dislocations and Mean Number of Dislocations per Patient.
      GroupsDislocations
      SumMean ± SDRange
      Anterior291.5 ± 0.7
      Kruskal-Wallis test, Mann-Whitney U-test (significance), the significance level is P < .05 with post hoc Bonferroni correction (adjusted significance), the anterior group had significantly less total number of dislocations compared to the lateral (P = .29) and the posterior (P = .023) group.
      1.1-1.8
      Posterior772.1 ± 1.01.8-2.5
      Lateral462.4 ± 1.61.6-3.2
      Total1522.0 ± 1.21.8-2.3
      SD, standard deviation.
      a Kruskal-Wallis test, Mann-Whitney U-test (significance), the significance level is P < .05 with post hoc Bonferroni correction (adjusted significance), the anterior group had significantly less total number of dislocations compared to the lateral (P = .29) and the posterior (P = .023) group.
      Figure thumbnail gr3
      Fig. 3Mean number of dislocations per patient between groups. ∗Kruskal-Wallis test, Mann-Whitney U-test (significance), the significance level is P < .05 with post hoc Bonferroni correction.
      Figure thumbnail gr4
      Fig. 4Number of dislocations and the number of patients affected.
      The approach was associated with direction of instability. Nearly all first dislocations in the AA were anterior dislocations (18/20, 90%). The posterior approach had 94% posterior dislocations (34/36). In the lateral approach 68% dislocated anteriorly (13/19) (P < .001).

       Timing of Dislocations: First Dislocation

      The time to first dislocation had a median of 44 days (range 0-1716). Twenty percent (15/75) of THA dislocations occurred within 2 weeks of the index THA (Table 3). Forty percent (8/20) of the anterior group dislocated within 2 weeks, compared to 14% (5/36) in the posterior group (odds ratio [OR] 4.1, 95% confidence interval [CI] 1.1-15.2, P = .04) and 11% (2/19) in the lateral group (OR 5.7, 95% CI 1.0-31.5, P = .04) (Table 4). Only 10% (2/20) of THAs in the anterior groups dislocated after 6 months compared to 25% (9/36) in the posterior group and 32% (6/19) in the lateral group.
      Table 3Number of Dislocations Based on Time Intervals After pTHA.
      Groups2 wk6 wk6 mo
      <2 wk>2 wk<6 wk>6 wk<6 mo>6 mo
      Anterior812128182
      Posterior5311719279
      Lateral217811136
      pTHA, primary total hip arthroplasty.
      Table 4OR of Dislocating Based on Time Intervals After pTHA.
      Comparison<2 wk>6 wk>6 mo
      ORSignificanceORSignificanceORSignificance
      Anterior-posterior4.10.04
      Significance set at P < .05 and based on Fisher’s exact test.
      0.60.260.30.16
      Anterior-lateral5.70.04
      Significance set at P < .05 and based on Fisher’s exact test.
      0.50.210.20.10
      Posterior-lateral1.40.540.80.470.720.41
      OR, odds ratio; pTHA, primary total hip arthroplasty.
      a Significance set at P < .05 and based on Fisher’s exact test.

       Natural History of Instability

      Of the 75 patients, 43% (32/75) had a successful closed reduction and 4% (3/75) had revision surgery after the first dislocation (Fig. 5). All hips that underwent revision surgery (n = 3; 2 posterior group, 1 lateral group) following the first dislocation did not dislocate further.
      Figure thumbnail gr5
      Fig. 5Flow diagram of the natural history of THA dislocation.
      Forty-three of the 72 THAs (59%) that had closed reduction sustained a second dislocation. The odds of having a second dislocation were significantly higher with the posterior (24/36; OR 5.2, 95% CI 1.2-11.7) and the lateral (12/39; OR 3.7, 95% CI 0.9-11.8) approach compared to the AA (7/20) (P = .02). Median time to second dislocation was 37 days (range 1-5408). The second dislocation took place significantly earlier in the anterior group (median 10 days, range 1-53) compared to the posterior (median 70 days, range 0-1849) (P = .009) and the lateral (median 36 days, range 1-5408) groups (P = .01).

       Timing of Dislocations: Multiple Recurrent Dislocation (>2)

      Of the 43 THAs that had a second dislocation, 63% (27/43) had further dislocations. Multiple dislocations (>2) occurred in 10% (2/20) of the unstable THAs of the AAs, compared to 47% (17/36) in the posterior group (OR 8.1, 95% CI 1.6-39.9, P = .004) and 42% (8/19) in the lateral group (OR 6.5, 95% CI 1.2-36.6, P = .03) (Table 5, Fig. 6).
      Table 5OR of Dislocating Anteriorly vs Posteriorly Based on Approach.
      ComparisonAnterior ApproachPosterior ApproachLateral Approach
      ADPDOR/SignificanceADPDOR/SignificanceADPDOR/Significance
      Second dislocation6/181/22.0/0.591/223/342.1/0.568/134/61.3/0.62
      >2 Dislocations2/180/20.9/0.811/216/340.9/0.734/134/64.5/0.16
      AD, anterior dislocation; PD, posterior dislocation; OR, odds ratio.
      Significance set at P < .05 and based on Fisher’s exact test.
      Figure thumbnail gr6
      Fig. 6Percentage of primary THA experiencing recurrent dislocations and revision surgery based on surgical approach. ∗Significance set at P < .05 based on Pearson chi-squared and Fisher’s exact test.

       Revision Surgery

      There were a total of 82 revision surgeries with a mean of 0.8 ± 0.8 revisions per patient. Thirty percent (6/20) of patients from the anterior group needed revision surgery, in contrast to 69% (25/36) in the posterior (OR 5.3, 95% CI 1.6-17.4, P = .005) and 68% (13/19) in the lateral group (OR 5.1, 95% CI 1.0-15.2, P = .02) (multivariate analysis, P = .01). There were no significant differences in odds of requiring revision surgery to achieve stability between the posterior and lateral groups (OR 0.9, 95% CI 0.2-2.4, P = .6). The mean number of revisions per patient with hip instability was significantly lower in the anterior group (0.4 ± 0.6) compared to the posterior group (0.8 ± 0.1; P = .008) and the lateral group (1.1 ± 0.3; P = .01). There were no significant differences in number of revisions between the posterior and lateral groups (P = 1.0). The large majority (57%) of revisions were head and liner exchanges. There was no difference in the type of revision for the different approaches (Table 6).
      Table 6Revision Operations for Dislocation (Cumulative).
      GroupsSumMean ± SDMinorMajorConstrained Liners UsedDual Mobility/Tripolar Used
      Head and LinerCup OnlyStem OnlyCup and Stem
      Anterior70.4 ± 0.6
      Kruskal-Wallis test, Mann-Whitney U-test (significance), the significance level is P < .05 with post hoc Bonferroni correction, significantly lower mean number of revision in the anterior group vs the posterior and lateral group.
      312102
      Posterior320.8 ± 0.116100691
      Lateral191.1 ± 0.31162051
      Total440.8 ± 0.8261337144
      SD, standard deviation.
      a Kruskal-Wallis test, Mann-Whitney U-test (significance), the significance level is P < .05 with post hoc Bonferroni correction, significantly lower mean number of revision in the anterior group vs the posterior and lateral group.

      Discussion

      Instability following THA remains a significant, common (0.2%-10%) complication [
      • Charissoux J.L.
      • Asloum Y.
      • Marcheix P.S.
      Surgical management of recurrent dislocation after total hip arthroplasty.
      ,
      • Berry D.J.
      • Von Knoch M.
      • Schleck C.D.
      • Harmsen W.S.
      Effect of femoral head diameter and operative approach on risk of dislocation after primary total hip arthroplasty.
      ,
      • Parvizi J.
      • Picinic E.
      • Sharkey P.
      Revision total hip arthroplasty for instability: surgical techniques and principles.
      ], most commonly occurring within 2 months of surgery [
      • Blom A.W.
      • Rogers M.
      • Taylor A.H.
      • Pattison G.
      • Whitehouse S.
      • Bannister G.C.
      Dislocation following total hip replacement: the Avon Orthopaedic Centre experience.
      ]. In this study, we describe the contemporary natural history of instability post-THA and present evidence illustrating that surgical approach is associated with differences in the natural history of instability post-THA. Dislocations following AA are likely to occur early and commonly within 2 weeks (over 40%) post-THA; late dislocations were seldomly seen. Furthermore, success of closed reduction differed between groups; the risk of having a second dislocation, after closed reduction of the first dislocation, was 3.7 times higher in the lateral group and 5.2 times greater in the posterior group compared to the anterior group. The odds of having multiple recurrent dislocations (>2) was significantly higher in the posterior (OR 8.1) and the lateral group (OR 6.5) compared to the anterior group. The odds of needing revision surgery was significantly higher in the posterior group (OR 5.3) and the lateral group (OR 5.1) compared to the anterior group.
      The literature has seldomly looked at the natural progression of hip instability in THA. Forsythe et al [
      • Forsythe M.E.
      • Whitehouse S.L.
      • Dick J.
      • Crawford R.W.
      Functional outcomes after nonrecurrent dislocation of primary total hip arthroplasty.
      ] reviewed the functional outcomes of pTHA that experienced a single dislocation treated with closed reduction and noted a trend of better quality of life and satisfaction in the control group that did not experience a dislocation. Chandler et al [
      • Chandler R.W.
      • Dorr L.D.
      • Perry J.
      The functional cost of dislocation following total hip arthroplasty.
      ] found that patients who had dislocated on several occasions walked significantly slower and had a reduced single limb support time. In a more recent literature, Kotwal et al studied the natural history of dislocation after pTHA and found that patients who dislocated more than once had worse outcomes compared to those who had a single dislocation episode. Furthermore, Barrack [
      • Barrack R.L.
      Dislocation after total hip arthroplasty: implant design and orientation.
      ] showed that dislocation of THA was associated with a higher mortality rate, and that functional and financial consequences are often underestimated [
      • Devane P.A.
      • Wraighte P.J.
      • Ong D.C.G.
      • Horne J.G.
      Do joint registries report true rates of hip dislocation?.
      ,
      • Barrack R.L.
      Dislocation after total hip arthroplasty: implant design and orientation.
      ].
      Our study showed that 40% of the AA pTHA dislocations happen within 2 weeks of surgery. The AA was performed with a partial capsulectomy technique and might be the reason why the dislocations were early. There were very few AA dislocations seen after 6 months, which might indicate that the capsule had adequate time to heal, offering additional anterior stability. In 2019, Lu et al [
      • Lu Y.
      • Wu Z.
      • Tang X.
      • Gu M.
      • Hou B.
      Effect of articular capsule repair on postoperative dislocation after primary total hip replacement by the anterolateral approach.
      ] showed less anterior dislocations in their study of anterolateral approach THAs when the anterior capsule was repaired. A recent prospective study showed no clinical differences, including dislocation rate, between resection and preservation of the anterior capsule with the AA [
      • Vandeputte F.J.
      • Vanbiervliet J.
      • Sarac C.
      • Driesen R.
      • Corten K.
      Capsular resection versus capsular repair in direct anterior approach for total hip arthroplasty: a randomized controlled trial.
      ]. Further research is needed to determine whether AA capsular repair would further decrease the dislocation rate in AA THA.
      Multiple factors may play a role in explaining why the AA has lower odds of having recurrent instability requiring revision surgery. Structures such as the abductors, the short external rotators, and the posterior capsule have all been identified as having significant contributions to hip stability and are disrupted during the posterior and lateral approaches [
      • Rogers M.
      • Blom A.W.
      • Barnett A.
      • Karantana A.
      • Bannister G.C.
      Revision for recurrent dislocation of total hip replacement.
      ,
      • Parvizi J.
      • Picinic E.
      • Sharkey P.
      Revision total hip arthroplasty for instability: surgical techniques and principles.
      ,
      • Conroy J.L.
      • Whitehouse S.L.
      • Graves S.E.
      • Pratt N.L.
      • Ryan P.
      • Crawford R.W.
      Risk factors for revision for early dislocation in total hip arthroplasty.
      ,
      • Pellicci P.M.
      • Bostrom M.
      • Poss R.
      Posterior approach to total hip replacement using enhanced posterior soft tissue repair.
      ,
      • Blom A.W.
      • Rogers M.
      • Taylor A.H.
      • Pattison G.
      • Whitehouse S.
      • Bannister G.C.
      Dislocation following total hip replacement: the Avon Orthopaedic Centre experience.
      ,
      • Lu Y.
      • Wu Z.
      • Tang X.
      • Gu M.
      • Hou B.
      Effect of articular capsule repair on postoperative dislocation after primary total hip replacement by the anterolateral approach.
      ,
      • Li E.
      • Meding J.B.
      • Ritter M.A.
      • Keating E.M.
      • Faris P.M.
      The natural history of a posteriorly dislocated total hip replacement.
      ,
      • Melvin J.S.
      • Karthikeyan T.
      • Cope R.
      • Fehring T.K.
      Early failures in total hip arthroplasty - a changing paradigm.
      ,
      • Lübbeke A.
      • Kampfen S.
      • Stern R.
      • Hoffmeyer P.
      Results of surgical repair of abductor avulsion after primary total hip arthroplasty.
      ,
      • Guo L.
      • Yang Y.
      • An B.
      • Yang Y.
      • Shi L.
      • Han X.
      • et al.
      Risk factors for dislocation after revision total hip arthroplasty: a systematic review and meta-analysis.
      ]. In comparison, disruption of the anterior capsule, whether it be iatrogenic (ie, capsulectomy in AA) or traumatic (anterior dislocation in AA), has not been associated with instability [
      • Odak S.
      • Ivory J.
      Management of abductor mechanism deficiency following total hip replacement.
      ] to date, although further research is necessary as the importance and contribution of the various ligamentous constituents of the hip capsule are being better understood [
      • Logishetty K.
      • van Arkel R.J.
      • Ng K.C.G.
      • Muirhead-Allwood S.K.
      • Cobb J.P.
      • Jeffers J.R.T.
      Hip capsule biomechanics after arthroplasty: the effect of implant, approach, and surgical repair.
      ,
      • Ng K.C.G.
      • Jeffers J.R.T.
      • Beaulé P.E.
      Hip joint capsular anatomy, mechanics, and surgical management.
      ]. Moreover, AA might have more consistent component orientation since studies have shown that THA done in the supine position (standard for AA) as opposed to the lateral decubitus position (standard for posterior and lateral approaches) had more consistent cup orientation [
      • Grammatopoulos G.
      • Gofton W.
      • Cochran M.
      • Dobransky J.
      • Carli A.
      • Abdelbary H.
      • et al.
      Pelvic positioning in the supine position leads to more consistent orientation of the acetabular component after total hip arthroplasty.
      ]. Use of intraoperative radiographs or fluoroscopy has also been found to improve cup orientation [
      • Grammatopoulos G.
      • Gofton W.
      • Cochran M.
      • Dobransky J.
      • Carli A.
      • Abdelbary H.
      • et al.
      Pelvic positioning in the supine position leads to more consistent orientation of the acetabular component after total hip arthroplasty.
      ,
      • Belyea C.M.
      • Lansford J.L.
      • Yim D.G.
      Utility of intraoperative fluoroscopic positioning of total hip arthroplasty components using a posterior and direct anterior approach.
      ,
      • Song J.-H.
      • Kim Y.-S.
      • Kwon S.-Y.
      • Lim Y.-W.
      • Jung J.
      • Oh S.
      Usefulness of intraoperative C-arm image intensifier in reducing errors of acetabular component during primary total hip arthroplasty: an application of Widmer’s method.
      ,
      • Ezzet K.A.
      • McCauley J.C.
      Use of intraoperative x-rays to optimize component position and leg length during total hip arthroplasty.
      ,
      • Delagrammaticas D.E.
      • Ochenjele G.
      • Rosenthal B.D.
      • Assenmacher B.
      • Manning D.W.
      • Stover M.D.
      Intraoperative evaluation of acetabular cup position during anterior approach total hip arthroplasty: are we accurately interpreting?.
      ]. In this study, use of intraoperative radiographs was not consistently performed as individual surgical practice varies.
      Many factors contribute to THA stability and the risk of dislocation [
      • Charissoux J.L.
      • Asloum Y.
      • Marcheix P.S.
      Surgical management of recurrent dislocation after total hip arthroplasty.
      ,
      • Blom A.W.
      • Rogers M.
      • Taylor A.H.
      • Pattison G.
      • Whitehouse S.
      • Bannister G.C.
      Dislocation following total hip replacement: the Avon Orthopaedic Centre experience.
      ,
      • Jones S.A.
      The prevention and treatment of dislocation following total hip arthroplasty: efforts to date and future strategies.
      ]. One of these factors, as described above, is the surgical approach. Historically, the posterior approach was reported to have higher dislocation rates compared to the other approaches, regardless of head size [
      • Padgett D.
      • Warashina H.
      The unstable total hip replacement.
      ]. However, modifications to the posterior approach in the form of an enhanced posterior capsular and soft tissue repair have lowered the dislocation rate from 4% to less than 1% [
      • Pellicci P.M.
      • Bostrom M.
      • Poss R.
      Posterior approach to total hip replacement using enhanced posterior soft tissue repair.
      ]. A recent systematic review revealed comparable dislocation rates in the anterior (anterolateral), direct lateral, and posterior approaches with soft tissue repair (0.70%, 0.43%, and 1.01%) [
      • Kwon M.S.
      • Kuskowski M.
      • Mulhall K.J.
      • Macaulay W.
      • Brown T.E.
      • Saleh K.J.
      Does surgical approach affect total hip arthroplasty dislocation rates?.
      ].
      The treatment of a dislocated THA is a challenging problem because there is a lack of consensus in the literature on the most appropriate management algorithm [
      • Sutphen S.A.
      • Lipman J.D.
      • Jerabek S.A.
      • Mayman D.J.
      • Esposito C.I.
      Treatment of recurrent dislocation after total hip arthroplasty using advanced imaging and three-dimensional modeling techniques: a case series.
      ]. An understanding of the underlying dislocation mechanism is essential to determining the treatment of instability. A thorough workup consisting of a history, physical examination, and imaging is required to identify a possible manageable etiology causing THA instability. When there is no gross modifiable etiology identified in a single episode of instability, the dislocation can be successfully treated with a closed reduction and a reinforcement of hip precautions. If improper alignment and hip mechanics are present, surgical treatment may be warranted. Approximately a third of patients with THA dislocations fail conservative measurements and require revision surgery [
      • Bozic K.J.
      • Kurtz S.M.
      • Lau E.
      • Ong K.
      • Vail D.T.P.
      • Berry D.J.
      The epidemiology of revision total hip arthroplasty in the United States.
      ]. This is reflected in our study showing 30% of the THA dislocations failing conservative management and requiring revision surgery. Common surgical management of THA instability includes the following: increasing femoral head size/offset, correcting malpositioned components, using elevated/lipped liners, dual-mobility constructs, constrained liners, and soft tissue repair [
      • Jones S.A.
      The prevention and treatment of dislocation following total hip arthroplasty: efforts to date and future strategies.
      ]. A more pragmatic approach would be to use preventative methods to avoid dislocations all together. Jones [
      • Jones S.A.
      The prevention and treatment of dislocation following total hip arthroplasty: efforts to date and future strategies.
      ] suggested the use of risk stratification to guide implant selection during pTHA. The findings of the present study can further add to this by suggesting that the AA be used for these “higher” risk patients in combinations with higher stability constructs.

       Limitations

      Some limitations include the fact that the study is a nonrandomized, retrospective chart review. There was an evolution of practice throughout the study period and an increase in use of AA. There was a learning curve associated with the surgeons implementing the AA in their surgical practice. There were more right-sided THAs in the lateral group and more left-sided THAs in the anterior/posterior groups. The patients in the anterior group were generally younger. There was a difference in head diameter between the groups. The decision to pursue revision surgery was based on the clinical judgment of the treating surgeon. There were no patient-reported outcome measures. All of these factors may contribute significant bias and introduce confounding variables that may have affected the results.

      Conclusion

      This study shows evidence that surgical approach influences the natural history of the unstable total hip replacement. Patients with pTHA performed through an AA who dislocate once and are treated with closed reduction will have lower odds of redislocating a second time compared to patients who had a pTHA performed through a direct lateral or posterior approach. The unstable pTHA done through an AA will also have lower odds of having multiple recurrent dislocation (>2) and a lower chance of needing revision surgery to achieve stability. Further, prospective or larger cohort studies are needed to confirm the present study's findings.

      Appendix A. Supplementary Data

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