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Risk Factors for Hip Dislocation After Total Hip Replacement

Posted on: 11/30/1999
What's every patient's fear after getting a new hip replacement? Dislocation. The surgeons call it instability. No matter what name you give it, the problem is troublesome for the patient and a complex challenge for the surgeon. In this review article, surgeons from the Shiley Center for Orthopaedic Research and Education in California offer surgeons insight into understanding what can cause this to happen and what to do about it.

There are two different scenarios that can happen around dislocation after a primary (first) hip replacement. The first is the timing: did it occur early or late after the surgery was done? The second is response to treatment -- more specifically, what to do when instability persists and doesn't respond to treatment.

The definition of early versus late dislocations isn't a time-frame agreed upon by everyone. Some say any instability after the first three months is a late event. Others pinpoint a much later time period such as five or more years after surgery. For sure, the later the dislocation, the greater the risk of recurrent dislocations unless revision surgery (a second operation) is done.

Studies show that most (two-thirds or more) of all dislocations after a primary hip replacement do, indeed, occur within the first year. There's a fairly even pattern in the number of patients affected during each quarter of the first year. So, if timing is not the issue, then what's causing these instabilities to develop?

Identifying risk factors for hip instability after primary total hip replacement is important in preventing these events. Reducing or minimizing risks is the first step to eliminating the problem altogether. It turns out there are quite a few possible risk factors such as older age, soft-tissue laxity (looseness), and weakness leading to loss of balance and falls resulting in dislocation.

Patients with other mental or physical problems are also at risk for hip joint instability. Alzheimer's, dementia, Parkinson's disease, alcohol and other drug abuse, and rheumatoid arthritis top the list. But there are some other risk factors that are less certain but for which some evidence has been presented. These include gender (female), body weight (being overweight), and decreased hip range-of-motion.

Many of these risk factors can't be changed (or at least not easily). That leads surgeons to look elsewhere to find ways of reducing the risk of instability after surgery. Their own work then comes under scrutiny. Does the operative approach (making the incision from the front, side, or back) make a difference? Are the type and size of implants selected part of the problem? Does the use of a plastic liner inside the joint or position of the implant make a difference? What about length and depth of the incision?

Here's what the evidence from studies shows so far:
  • Component size and design are very important. Larger femoral heads dislocate less often.
  • Neck shape and length (longer) and socket depth (deeper) also make a difference.
  • Making the incision along the front and side called an anterolateral approach has the lowest rate of dislocation.
  • A posterior approach can be done successfully (without dislocation) IF the joint capsule is repaired carefully.
  • Elevated liners placed between the cup (socket portion of the implant) and the femoral head decrease the risk of dislocation. If they are too thick, impingement (pinching) of the femoral neck against the socket can occur causing pain and loss of motion.
  • Placement of the cup side of the implant is important. It there's too much tilt or the cup orientation is too far off-center, dislocation is more likely. There is a safe zone in terms of how much leeway there is for both of these factors.

    Other risk factors that have been called into question include patient compliance with postoperative directions and the use of minimal incision and computer-assisted surgery. Regarding patient cooperation in following the surgeons directions not to bend or twist the hip too far -- no one knows for sure whether these restrictions are really needed or helpful. Some surgeons have experimented with not using strict postoperative guidelines on movements and activities with good results. There's a need for research in this area.

    Minimal incision and computer-assisted surgeries are fairly new. Early results of studies done show that neither of these procedures increases the risk of hip dislocation. If anything, they should actually reduce the risk. In theory, smaller incisions that don't disrupt the muscles should be a benefit. Likewise, computer-assisted navigation should make it possible to place the implant more accurately. Time will tell.

    The authors also offer some insights into treatment after dislocation occurs. The first step is to figure out why the dislocation occurred in the first place. Treating the cause of the instability is as important as fixing the problem. The surgeon must consider both patient and operative factors. X-rays and other imaging studies may help.

    Conservative (nonoperative) care is usually tried first. Once the hip is back in the socket, a brace is applied and the patient is sent to physical therapy. The therapist helps the patient learn how to move safely while strengthening the muscles around the hip. A special focus of treatment is to restore normal joint proprioception (sense of joint position) and kinesthesia (awareness of movement). Balance training is also very important.

    If that doesn't work and the hip dislocates again and again, then surgery may be needed to tighten up loose tissue and restore a balance to soft-tissue tension on all sides of the hip joint. If the implant is improperly positioned, it must be removed and realigned. The surgeon may need to replace the femoral head with a larger one. Sometimes adding a liner (or exchanging for a thicker liner) is all that's needed to offset the joint and prevent dislocation.

    The authors offer surgeons many possible choices during revision and reasons why each one might be considered. There is a lengthy discussion on femoral head size, choosing the proper liner, and implant positioning as possible ways to prevent a dislocation after a primary total hip replacement (or for revision surgery).

    They conclude by saying that understanding the risk factors (especially those the surgeon has control over) is an effective way to prevent instability and/or treat it once it occurs. Further studies are needed to look at other possible contributing factors such as the use of patient restrictions after surgery.

  • References:
    Clifford W. Colwell, Jr. Hip Reconstruction. Instability After Total Hip Arthroplasty. Current Orthopaedic Practice. January/February 2009. Vol. 20. No. 1. Pp. 8-14.

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