The purpose of this paper is to assess clinically and radiologically our experience with constrained acetabular components for the unstable hip following total hip replacement. From July 2006 to August 2012 we retrospectively reviewed the clinical and radiographic outcome of 22 arthroplasties, in 20 patients. The mean age at surgery of constrained acetabular component was 73 years (range, 35 to 90 years) and the mean clinical and radiological follow-up period was 35 months (range, 3 to 73 months). Clinical assessment was performed by the Harris Hip Score and the SF-12. The constrained acetabular device eliminated or prevented hip instability in all patients except in 3 hips who had new dislocation. The mean Harris hip score in the last evaluation was 74 points, and SF-12 was 27 points. A constrained acetabular components are simple to use and provides satisfactory mid term results for the treatment of hip instability in primary and revision replacement in those at high risk of dislocation. The potential for aseptic loosening requires evaluation by long term studies and bigger series.

Coll GF, Martí MB, Planell RV, Montfort ER, Fernàndez RS, et al. (2014) Unstable total Hip Replacement. are Constrained Liners A Solution? Experience and Literature Review. Ann Orthop Rheumatol 2(4): 1033.

Acetabular, Constrained liner, Hip Instability

Dislocation is one of the most common complications in total hip arthroplasty, ocurring after 0,4 – 7% of primary procedures and up to 19% of revisions. Various predisponing causes and associated factors have been identified. These include malposition of the components, previous surgery and revision THA, gluteal muscle deficiency, trochanteric nonunion, bone or cement impingement, soft-tissue imbalance, neuromuscular disorders. Surgical options for treatment include revision the position of the components, trochanteric advancement, capsulorraphy, improvement of soft-tissue tensioning by increasing offset, augmentation of the acetabular lining, bipolar arthroplasty, insertion of a constrained cup mechanism, inverted cup prosthesis, Bousquet’s acetabular component.

At our hospital constrained acetabular components have been used in four situations: [1] cases wich no identifiable cause of instability, [2] cases with surgical attempts at stabilization have failed, [3] cases with significant deficiency of soft-tissue attachments around the hip and [4] patients with neuromuscular disorders.

The use of constrained acetabular components has gained interest in the last 15 years with multiple short term papers reporting success rates of greater than 80%.

Some surgeons believe increased mechanical stresses inherent to these designs may predispose these hips to increased risks of wear, osteolytic processes, loosening and potential dissociation of the components with subsequent re-operation.

From July 2006 to August 2012 we retrospectively reviewed the clinical and radiographic outcomes of 22 arthroplasties, in 20 patients, 8 women and 14 men. The right side was the predominant with 14 cases.

Of the 22 patients who were identified, 3 patients had been lost because they died for causes unrelated to the arthroplasty. In 18 hips the constrained component was implanted for the treatment of recurrent instability (Figure 1a and 1b), two hips was implanted because grossly deficient soft-tissue attachments were believed to be associated with a high risk for subsequent instability, and in two cases were indicated for neuromuscular disorder, West Syndrome.

The primary indications for total hip replacement were arthrosis in 13 cases and femoral neck fracture in 8 cases and, one case of comminuted per-trochanteric fracture.

The mean age at surgery was 73 years (range , 35 to 90 years) and the mean clinical and radiological follow-up period was 35 months (range, 3 to 73 months). Clinical assessment was performed by the Harris Hip Score and the SF-12. All radiographs were evaluated for evidence of loosening.

The surgical approach was posterolateral in 9 hips and anterolateral in 13. Including the initial arthroplasty, all patients had undergone at least one reconstructive operation on the hip prior to insertion of constrained cup. Table 1 provides an overview of the patient demographics and follow-up details. A constrained liner is a device consisting of a liner and reinforcing ring. The polyethylene liner fits into the acetabular metal shell and articulates with the femoral head. The articular surface opening of the constrained liner is reduced slightly to allow for mechanical capture of the femoral head. The constrained acetabular components used in our series consisted in constrained cementless acetabular component inserted in a press-fit fashion. The metall-back component was secured with three screws at least. In two hips a polyethylene constrained insert was cemented into a well-fixed cup. Four types of constrained acetabular liner were used in the study; The trilogy Acetabular System Constrained Liner (Zimmer) was used in 8 cases, the RingLoc II Constrained (Biomet Orthopedics) was used in 9 hips, one case with Smith & Nephew liner and one hip with Duraloc Constrained Liner (DePuy). Anteroposterior and lateral radiographs of the hip joint were revised to assess the position of the implant and look for signs of loosening or wear prosthesis. Acetabulum was divided into three zones (De Lee and Charnley zones) to evaluated the presence of radiolucent lines. Possible loosening was defined as the location of a radiolucent line occupying >50% but <100% of bone-component interface on any radiograph or the presence of a progressive radiolucent line.

The constrained acetabular device eliminated or prevented hip instability in all patients except in 3 hips who had new dislocation. The mean Harris hip score in the last evaluation was 74 points (range 33-96), and SF-12 was 27 points (range 12-48).

There were 2 cases of disassembly and one case of dislocation of the hip components (Figure 2). All of them had a deep infection. Three hips more needed a superficial debridement. Aditional complications in the remaining cases included heterotopic ossification (two hips), urinay infection (three hips). There was no deep venous thrombosis and no ciatic nerve palsy.

There were three cases with radiolucent lines, two in zone I and one in zone II, but only one of them was revised. This case was revised because radiolucent lines were progressive.

Recurrent instability is often multifactorial and frequently is not associated with a clearly identifiable cause [1,2]. It has been estimated that surgical treatment of recurrent instability is required after approximately 1% of total hip arthroplasties [3,4,5]. Chronic instability occurs in up to 33% of dislocations [6,7,8]. Before the use of constrained liners, there were no reliable solutions to dislocation arising from inadequate soft tissues, a deficient abductor mechanism, or neuromuscular disorders [9].

The use of constrained acetabular components has gained interest over the past decade, with multiple short-term studies reporting success rates of greater than 80% [5]. The risk of dislocation following revision is increased in comparison to primary procedures. As a result of this high failure rate for these patients, the use of a constrained acetabular component has increased [4,5]. William JT Jr et al. analized eight papers included 1199 hips in 1148 patients with a total mean follow-up of 51 months. The mean rate of dislocation following revision with a constrained liner was 10% and the reoperation rate for reasons other than dislocation was 4% [5]. In a retrospective study involving clinical and radiographic outcome of 110 arthroplasties using a constrained acetabular component the device eliminated or prevented hip instability in more than 98% of cases [2]. Goetz et al., showed fifty-four (96%) of fifty-six hips stable after insertion a constrained acetabular device [10]. A review of the literature of use of constrained acetabulars components are shown in Table 2.

The managament of hip instability with a cemented or uncemented, constrained acetabular component is useful when there is no single identifiable cause of instability but, there are a number of potential problems with the use of them. [1,2,11] The use of cemented component include premature acetabular loosening, rapid polyethylene wear, decreased range of motion [1,9,6,12,13]. However the use of a cemented liner eliminates backside wear and the direct access of wear particles to the acetabular bone stock, which may help to reduce the potential for acetabular osteolysis [1].

Khan et al. analyzed 107 hips in 104 patients treated with the uncemented constrained cup. There was an inverse association between the number of srews used to secure the cup and subsequent loosening, which almost reached statistical significance. Cups with no screws or a single screw were more likely to loosen. There was no siginficant association between component migration or loosening and age, sex, cup size, the use of bone graft, or position of the shell in the vertical and horitzontal planes [14]. Our point of view is that use of bone graft, hinders the well-fixed cup, and vertical position of the constrained shell brings on loosening component. Bakker et al. have published that implantation of the acetabular component in the correct orientation is critical in preventing impingement and subsuquent stresses on the interfaces of acetabular reconstruction [1].

Complications of constrained acetabular components can be devided into 3 categories. The first category is directly related to the constraining mechanism such as dislocation, head dissociation from de stem, liner dissociation from the acetabular device. The second category is related to increased constraint such as aseptic loosening and osteolysis, and periprosthetic fracture. The third category includes infection, deep vein thrombosis, ciatic palsy and periprosthetic fracture [5,15,11].

Pattyn C. et al. recommend judicious use of contrained devices because of high failure rate (26%) and consideration of alternative options such as the use of large-diameter metal-onmetal bearings [15,16]. A current study with minimum 10-year follow-up of 667 hips, the recurrent dislocation rate was 17,5% [15]. The failure rate directly related to the constraining device, including dislocation, was 18,6%. They conclude that use these components do not appear to decrease the rate of dislocation from the 19% rate previously described for revision surgery [13]

Great concerns exist regarding the quality and thickness of the polyethylene used in these devices. For example the liners used in the majority of cases were frequently coupled with 32 femoral heads resulting in a polyethylene thickness of 3.0 to 5.0 mm [15]. Similar catastrophic wear has been referenced by others [17,18]. Newer products with improved polyethylene quality, manufacturing, and sterilization techniques, in addition to novel constraining mechanisms, may provide increased durability, improved longevity, and decreased wear [15,19].

Regarding the radiolucencies many of the hips that had had multiple operations frequently had associated acetabular bone deficiencies. Some of the radiolucencies that were observed may have been a consequence of this factor rather than a result of any effect caused by the contrained acetabular insert [2].

In cases of hip instability with a well-fixed cementless acetabular shell, cementing an acetabular liner into secure attached cup could be a great solution [20]. A 2-mm cement mantle was adequate for fixation . However Saphiro et al. and Su et at do not recommend this mode of fixation because they reported several hips which required revision for aseptic loosening before 20 months of follow-up [9,6].

Other designs of device to avoid the dislocation have been found in literature. We have mentioned theese we believe more relevant. The first is the constrained tripolar design which consists of a femoral head that snaps into a polyethylene shell with a polished cobalt-chrome backing, which in turn articulates within an other polyethylene liner. This bipolar construction is then snapped into a Osteonics (Stryker) acetabular shell [5,9,21]. The second device is a custom-made constrained construct which consists of the application of an inverted cup over the femoral neck or head, the cup being fixed to the acetabular insert with screws [22]. The third model is the Bousquet’s acetabular component that is an steel cup covered with alumina, impacted without cement. The polyethylene component is free in the cup and retentive on the femoral head [23].

Table 1: Demographic Information.

Table 2: Clinical outcomes of constrained acetabular components.

A constrained acetabular components are simple to use and provides satisfactory mid term results for the treatment of hip instability in primary and revision replacement in those at high risk of dislocation. They are a reasonable and reliable method for restoring stability at the site of a complex unstable hip replacement.

The potential for aseptic loosening requires evaluation by long term studies and bigger series.

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17. Dearborn JT, Murray WR. Arthopor 2 acetabular component with screw fixation in primary hip arthroplasty: a 7- to 9-year follow-up study. J Arthroplasty. 1998; 13: 299-310.

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