For hip resurfacing arthroplasty prioritize exposure and cup insertion

by Thomas Parker Vail, MD

The overarching concept for successful hip resurfacing is to optimize the intended bearing function and minimize the chances for unintended patterns of movement like subluxation or edge-loading. This goal can be realized with specific strategies for cup implantation as well as overall biomechanical hip reconstruction. When using a monoblock cup as part of a hip resurfacing or total hip procedure, it is important to note that a monoblock cup may be more challenging to insert than a modular cup with a dome hole and screw holes.

With respect to the technique of hip resurfacing, the goal of the overall construct should be to achieve a congruency of coverage of the socket component over the femoral head resurfacing component. In resurfacing, the femoral and acetabular components are linked by size constraints. Because the outer diameter of the femoral head component must match the inner diameter of the monoblock acetabular shell, there may be only one (or possibly two) acetabular component sizes available for a given femoral component in most implant systems.

The overall biomechanical reconstruction of the hip requires that the relationship of the femoral head component and acetabular shell be optimized to reconstruct offset and length appropriately and promote optimal bearing function. With respect to bearing function, a great deal of attention has been paid to the position of the acetabular shell. Optimal placement of the acetabular shell is achieved by placing the hip center close to the anatomic center and avoiding extremes of abduction and anteversion.

Avoid edge loading

The goal of hip reconstruction is to optimize the function of the bearing articulation; therefore, any combination of implant positions that could increase the chance for edge loading is less desirable. Implant positions that lead to a lack of offset caused by excess femoral-side valgus or shortening of the femoral neck can increase the chances for extra-articular impingement and/or soft tissue laxity. When laxity or impingement occurs in combination with a more vertically placed acetabular component, the combination of circumstances will amplify the negative contribution of cup position toward the undesired outcome of edge loading. Thus, the overall biomechanical reconstruction is as important as proper acetabular positioning.

Thomas Parker Vail

To state the situation from a somewhat different perspective, if the cup position is only slightly more vertical or anteverted than desired, but laxity or impingement predispose to subluxation, then there will be an increased chance that the femoral component will load the anterior or lateral edge of the acetabular component. Edge loading can damage the component and increase local metal levels, with negative consequences for some patients.

Careful patient selection

To achieve optimal results, it is important to choose the right cases for hip resurfacing and recognize those patients who would benefit more from total hip replacement. The right cases are those with end stage articular degeneration in the setting of good bone quality with close-to-normal hip mechanics.

Given the limits of reconstruction options when using hip resurfacing techniques — particularly for bone loss on the femoral side — it is best to avoid cases with dramatic alterations in joint mechanics. While there is some latitude to move the hip center by positioning of the acetabular component, the femoral component must be placed where the bone is available to support the implant. Thus, in cases with a dramatically altered hip center, leg length discrepancy or bone loss, the normal hip mechanics will be difficult to restore with a hip resurfacing implant.

Component positioning can be planned by using standing films as a template to determine cup position. In particular, take note of the medial osteophyte when planning the acetabular position, and target the hip center, noting the relationship of the lateral edge of the acetabular bone rim to the prosthetic cup edge.

Exposure

I use a posterior approach when performing hip resurfacing. The posterior approach incision is situated slightly posterior to the trochanter — a bit more posterior than my standard total hip incision. I generally make a curved incision from just below to just above the tip of the trochanter. The interval beneath the gluteus medius is then determined. A retractor is placed there, exposing the piriformis tendon and short external rotator muscles.

Component position and restoration of hip mechanics are important for optimal implant function in hip resurfacing. Image: Vail TP

It is critical to try to protect the blood supply to the femoral head. Thus, a robust cuff of tissue is preserved around the base of the neck. The lateral shoulder of the proximal femur and piriformis fossa are not skeletonized or exposed as they may be in total hip arthroplasty. The blood supply is entering the femoral head and neck on the posterior and lateral side of the neck of the femur, branching off the medial femoral circumflex artery posteriorly.

To facilitate movement of the femur for exposure of the acetabulum, the anterior hip capsule is elevated sharply and a retractor is placed underneath the femoral neck and over the anterior rim of the acetabulum. This maneuver allows full exposure of the acetabulum, including visualization of the transverse acetabular ligament. For further exposure, the inferior capsule can be divided radially with care to avoid obturator artery branches. Excision of redundant capsule and damaged labrum will discourage soft tissue in-folding, allowing greater ease of cup insertion.

Potential pitfalls during the exposure are as follows:

• placing the incision too far anterior makes it difficult to access the socket;
• stripping the soft tissues can potentially impact the blood supply; and
• inadequate mobilization of the femur will push your reamer posteriorly.

Use your standing film to judge the abduction opening angle of the acetabulum to give yourself a target during surgery. It is especially important to note that, when looking at the acetabulum from the posterior approach, the 12 o’clock position is not the lateral projection. A cup placed evenly with the 12 o’clock position will be too vertical, and vertical cups lead to higher wear and higher metal ion levels.

For proper acetabular positioning, I use the transverse acetabular ligament, setting the anteversion first and then the abduction. If done in reverse order, it is more likely that the cup will end up in a position that is too vertical. Finally, the anterior edge of the cup should be below the rim of the acetabulum to discourage extra-articular impingement.

The position of the reamer in the acetabulum can be used to provide a visual clue to component seating and cup position. Because it is possible to see through the reamers, and it is not possible to see through a monoblock cup, use the reamers to note the “fully seated” position of cup edges relative to the local bone anatomy.

Standard concentric reaming is utilized. The most peripheral teeth on the reamer often will not extend to the lateral edge of the acetabular bone. If this is the case, it is helpful to move the reamer concentrically to be certain that an overhanging peripheral rim of bone has not been left behind which could impede acetabular cup implantation.

For implantation, place the cup inside the soft tissue envelope, making sure that it is in contact with bone before striking it. If soft tissue is interposed, the cup will never go down. Place the cup just below the anterior rim, with a few bead lines lateral to the lateral edge of the acetabulum, recalling the position of the last reamer.

Be certain to ream the lateral rim and know the fully seated position relative to your reamer. Because monoblock cups have no holes in them, you must know when the cup is fully seated based on its relationship with the bone at the periphery. If the cup is too vertical, not seated fully to the base of the reamed bed, overmedialized, perched on the rim, or over-reamed, the negative effect on the implant function can increase the potential for a revision.

Pitfalls to avoid

Avoid a vertical cup, an overly anteverted cup and an incompletely seated cup. If the peripheral bone rim is not adequately reamed relative to the dome, the cup will not seat past the prominent peripheral rim. An under-reamed bone or prominent rim can either deform the cup or prevent the cup from seating fully, thus causing the cup to sit more laterally than anticipated with less overall bone contact.

A cup design with a lateral flare or flanges will also change the amount of force required for seating compared with a standard spherical modular cup. Lastly, the cup will not seat properly if the soft tissue around the rim has not been excised and becomes in-folded between the cup and the bone.

References:

• Vail TP. Exposure, socket preparation and one piece cup insertion. Presented at the 2010 Annual Meeting of the American Academy of Orthopaedic Surgeons. March 9-13. New Orleans.

• Thomas P. Vail, MD, can be reached at the Department of Orthopedic Surgery, University of California, San Francisco, Box 0728, 500 Parnassus Ave., MU320W, San Francisco, CA 94143-0728; 415-502-5183; fax: 415-476-7174; e-mail: vailt@orthosurg.ucsf.edu. He has received royalties from and is a paid consultant or employee of Depuy, A Johnson & Johnson Company, he has received research or institutional support from Depuy, A Johnson & Johnson Company, Musculoskeletal Transplant Foundation, Stryker Corporation, Stryker Howmedica Osteonics, TissueLink Medical, Inc., and Zimmer, Inc.