Comprehensive Review of Current Constraining Devices in Total Hip Arthroplasty

Johannes Michiel Van der Merwe, MBChB, FRCSC


J Am Acad Orthop Surg. 2018;26(14):479-488. 

In This Article

Abstract and Introduction


Hip instability after total joint arthroplasty is a devastating complication. Appropriate management of instability is a challenge. Three components that are commonly used in these challenging scenarios are constrained liners, constrained tripolar components, and nonconstrained tripolar components. The biomaterials and biomechanics of these devices vary. Surgeons must take into account the risks associated with each of these components and some surgical pearls for their use. A thorough review of the recent literature allows comparison of results addressing the short-, medium-, and long-term survival of each component. Constraining devices are a good option when used in salvage procedures in elderly and/or low-demand patients with hip instability. However, constraining devices should not be used to correct deficiencies in surgical technique or implant placement.


Primary and revision total hip arthroplasty (THA) procedures are being performed more frequently, as shown in the Australian[1] and Swedish[2] national arthroplasty registries. Revision surgery for hip instability accounts for a small proportion of revision procedures.

Many studies have demonstrated instability rates ranging from 0.5% to 10% after primary THA and from 10% to 25% after revision hip surgery.[3] The cumulative risk of dislocation does not remain constant after THA but increases with time because of trauma, polyethylene wear, increased pseudocapsular laxity, and deteriorating muscle strength. The cumulative risk of dislocation after primary THA is 7% at 25 years.[3] The risk factors for instability are summarized in Table 1.

Three nonconstrained options are available to address hip instability: a nonconstrained large-diameter femoral head in a standard polyethylene liner, which is snap-fitted into an acetabular cup; a nonconstrained acetabular shell with a smooth metal bearing, articulating with a large polyethylene ball that holds a constrained and mobile femoral head; and a metal bipolar component articulating with a nonconstrained acetabular component with a snap-fit polyethylene liner. The latter is used when the surgeon encounters a wellfixed nonmodular femoral component with a small femoral head that will articulate with an oversized acetabular component. This mismatch between the large outside diameter of the acetabular shell and the small diameter of the femoral head will lead to a large dead space around the acetabular component into which the femoral head can dislocate.[4] For example, if the surgeon uses a 64-mm revision acetabular shell and a 28-mm femoral head, the dead space around the acetabular component will be 18 mm (ie, 64 mm/2 = 32 mm; 28 mm/2 = 14 mm; 32 – 14 mm = 18 mm). The soft tissues thus cannot provide adequate restraint to avoid dislocation into the large dead space. Instead of having a large cup-to-head diameter mismatch, the femoral component can easily be converted to a bipolar component, which can articulate with the acetabular component.

Two constraining options are available for a tripolar component: a standard hip arthroplasty implant with a locking ring maintaining the femoral head in the polyethylene liner, and a constrained tripolar component, which consists of two locking rings maintaining the large bipolar component in the acetabulum and retaining the femoral head within the bipolar component.

The main indications for the use of constraining devices are summarized in Table 2.[5–7] The difficulty arises in choosing the correct implant for the appropriate patient. In general, the surgeon should use the least amount of constraint needed to achieve maximal stability. Fully constrained systems cause excessive stresses on the different interfaces, which can lead to early failure. The surgeon should be prepared to adapt to a new surgical plan depending on intraoperative findings. A simple liner exchange procedure can easily become a more complex procedure requiring a constrained tripolar design, such as when soft-tissue damage that might lead to instability is encountered intraoperatively, as sometimes occurs in metal-onmetal revisions.

A constraining device should not be used to correct deficiencies in the surgical technique or component placement. The deficiencies should be addressed first, and if instability persists after correction of the abnormalities, a constraining device can be used as a last resort. Failure to address the anomalies first will result in early failure despite the use of a constraining device.