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THR technology, technique continue to evolve

Participants consider the characteristics of mobile-bearing designs, the clinical performance of the devices.

Issue: January 2004

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Technology for total hip replacement continues to evolve. There are developments in surgical approach and perioperative management, as well as in new materials, implant designs and instrumentation. The goals of the procedure are also evolving. While the quality of the long-term result continues to be a hallmark of success, the short-term result and the functional capacity of the arthroplasty are now priorities for many. Demand for the procedure and patient expectations are high, but reimbursement continues to fall. In this virtual round table discussion, leaders in the field weigh in on the issues.

— Thomas P. Schmalzried, MD
Moderator

Thomas P. Schmalzried, MD: What is having the greater impact on total hip replacement: implant technology or evolution of the surgical procedure?

Richard A. Berger, MD: We have spent the last two decades improving implant technology. While there may be radical changes in the future, it will be hard to significantly improve on the reported 10- to 15-year survival of more than 95%. Conversely, surgical technique has changed little in the last two decades. Furthermore, we are only recently concentrating on approaches to achieve a quicker recovery. Therefore, I believe improvements in surgical technique and perioperative care can and will have the greatest impact on arthroplasty in the next few years.

John J. Callaghan, MD: Implant technology, especially in relationship to bearing surface issues, should have the greatest impact on total hip replacement. I say this because most of us firmly believe that with the use of the cementless devices available today, fixation should not be an issue. Bearing surface wear with subsequent development of osteolysis is the long-term clinical problem.

Schmalzried: Does “mini-incision” total hip replacement substantially improve short-term outcomes?

Berger: Let’s differentiate a cosmetic procedure, a small incision with a standard dissection beneath, from a functional procedure, a small incision with minimal soft tissue disruption beneath. If we consider a functional procedure, such as the minimally invasive two-incision hip replacement, where no muscle or tendon is cut, I have found a substantial short-term pain and functional improvement over traditional hip replacement. In the last 100 patients done at Rush Hospital with this procedure, 97 have been performed as an outpatient. The average patient is off crutches in six days and walking independently by nine days. The average time to return to work is eight days.

Daniel J. Berry, MD: At the moment we do not have enough information to answer this question definitively. There certainly is evidence that a mini-incision in combination with accelerated rehabilitation protocols and modified anesthesia and postoperative pain management protocols allows selected patients to be dismissed from the hospital and return to activities earlier than once was thought possible. It has been difficult, however, to determine how much of this acceleration of recovery is related to patient selection; change of rehabilitation protocols; change of anesthesia and pain management protocols; and altered patient perceptions and expectations.

Despite these reservations, one can make the logical assumption that substantially reducing soft tissue trauma during surgery should reduce postoperative pain and allow quicker rehabilitation from the soft tissue viewpoint. Nevertheless, the short-term outcome of a procedure is not only the speed with which patients recover but also the quality of the arthroplasty provided and the rate of complications associated with that arthroplasty. A comparison of these parameters — even at short term — between minimally invasive and conventional hip arthroplasties is not yet available.

Schmalzried: How do you control for the “placebo effect” of small-incision surgery?

Berger: It is difficult to fully control for the placebo effect of any surgical procedure; however, studying patients who had a small incision with a standard dissection beneath provides some insight. Sculco reported such a cohort and found only a small measurable benefit of a mini-incision over a standard incision. This small improvement may indeed be the result of the placebo effect. However, the placebo effect seems unlikely to be responsible for the dramatic rapid recovery of patients who have undergone a truly minimally invasive procedure, where no muscle or tendon is cut.

William Maloney, MD: To control for the placebo effect in minimally invasive hip replacement surgery, one would have to do a randomized clinical trial. This clearly needs to be done. We know that with other medical interventions, especially those aimed at reducing pain, the placebo effect can be significant. In addition, patients undergoing minimally invasive hip surgery may have rather extensive preoperative conditioning as to the expectations for early return to function. Because patients in general want to please their surgeons, this may also play a role in their rate of recovery.

Schmalzried: Are the risks of mini-incision total hip replacement any different than those of the standard procedure?

Berger: As with any new procedure, until surgeons become proficient there will probably be more complications. This is why the advocates of these procedures, the American Academy of Orthopaedic Surgeons and industry must help train surgeons in these new techniques with hands-on and individual instruction. However, I believe that the risk of a minimally invasive surgery may eventually be less than a traditional approach. With less soft tissue disruption the incidence of dislocation, wound drainage and infection should be less.

Callaghan: If mini-incision total hip replacement is to prosper, the risks cannot be any different and especially cannot be greater than those associated with the standard operation. This must be the case for both innovators of the procedure and for the general orthopedic surgeons who will be required to service the ever-increasing numbers of patients afflicted with hip arthritis who will require total hip arthroplasty in the future.

If infection, fracture and dislocation rates are higher, if long term there are more failures of fixation and increasing wear (related to the use of more extended liners, larger heads, the inability to adequately clean and assemble modular connection and introduction of more third bodies into the bearing surface during head reduction) then the short-term “potential” benefits (cosmesis, early return to function) are not worth it.

Schmalzried: Is there an aggregate advantage favoring one of the currently available total hip bearings (eg, crosslinked poly, metal-metal and ceramic-ceramic)?

Callaghan: There is no definitely proven advantage of any one of the newer bearing surfaces available in total hip arthroplasty. Fortunately, none of the three are really new. All were used to some extent in previous generations of total hip arthroplasty. These bearings show promise in reducing bearing surface wear, but all have potential concerns like fatigue properties of crosslinked polyethylene, serum ion concentrations and impingement issues with metal-metal, and fracture and impingement issues with ceramic-ceramic.

Maloney: Each of the newer bearing surfaces in total hip replacement has pros and cons. There is no clear evidence that in aggregate that one has an advantage over the others.

Schmalzried: Is it appropriate to routinely use larger diameter bearings (32mm or larger)?

Berry: Larger diameter bearings are appealing because of the potential to decrease dislocation rates. Larger diameter bearings are now considered because new bearing surfaces may have low enough volumetric wear rates to allow use of large diameter bearings without a high rate of osteolysis. For crosslinked polyethylene bearings we hope that improved wear characteristics may offset any increased wear rate associated with larger head sizes, but to date, we have only very early information on how crosslinked bearings have performed in human beings. Therefore, it seems prudent in patients who are at risk for wear and osteolysis to stick with a conventional femoral head size even with crosslinked bearings until more data are available on how the crosslinked bearings are performing in vivo. On the other hand, in patients who are unlikely to have wear-related problems, and particularly in patients at high risk for instability, then using a larger femoral size would seem reasonable based on current data.

Lastly, when hard-on-hard surfaces are used, large heads provide the triple advantages of improving hip stability, decreasing impingement risk and reducing wear rate (because for hard-on-hard bearings, larger head sizes create better fluid film lubrication and reduce bearing wear).

Maloney: Larger diameter bearings certainly are popular. Their popularity relates to their real and perceived effect on stability after total hip replacement. Although wear, lysis and loosening are significant problems in the long run, the main problems in the short term are dislocation and infection. Dislocation rates correlate with surgical experience.

Total hip replacement in the United States is a general orthopedic procedure. Approximately 50% of the Medicare total hip replacements are done by surgeons who do 10 or fewer Medicare hip replacements per year.

Larger femoral heads increase the implant-on-implant impingement free range of motion, which in theory should decrease postoperative dislocation. In terms of routine use of larger femoral heads, one has to look at each bearing option independently. Metal-on-metal is better from a mechanical standpoint with larger heads, so with a metal-on-metal bearing the use of larger femoral heads is not an issue.

With ceramic-on-ceramic bearings, there are material limitations related to head size. Because there is a limit on how thin one can make the acetabular liners without increasing the risk of fracture, larger femoral heads can only be utilized with larger acetabular components. In one system, 36-mm heads are only available in cup sizes 64 mm and larger. With metal-on-conventional polyethylene, increasing the femoral head size has resulted in increased volumetric wear of the polyethylene. Laboratory data have demonstrated that with highly cross-linked polyethylene, femoral head size does not appear to be an important variable in wear. Time will tell, but the basic science data are quite strong.

Schmalzried: Has the evolution of implant design and larger bearings reduced the rate of hip dislocation? Primary and revision?

Berry: We now have evidence that 32 mm heads have a lower dislocation rate than smaller head sizes but we don’t know to what degree head sizes larger than 32 mm will further improve dislocation rates. One would anticipate there would be even greater improvements in larger head sizes, but this remains to be proven in clinical studies.

Many other implant design modifications may reduce dislocation rates, including improved acetabular liner design and improved femoral neck design (both of which are designed to increase prosthetic range of motion free of impingement); femoral implants with improved offset; and liners that provide slight lateralization or the ability to fine-tune acetabular component position without increasing intra-articular impingement (so-called face changing liners). Currently, there are no clinical data to substantiate the hope that these changes in prosthetic design will substantially reduce dislocation rates, but it seems likely that they will help.

Callaghan: The use of larger bearings in the range of 36 mm to 40 mm have not been available for long enough to know if they will clinically reduce dislocation; however, from a theoretical standpoint they should. Data are becoming available from long-term studies of dislocation, including those of Dr. Berry, which demonstrate a lower dislocation prevalence when 32 mm heads were utilized compared to when 28 mm and 22 mm heads were used. This difference is more striking in revision surgery when larger outer diameter shells are utilized.

Hence, in revision surgery and in cases with larger diameter acetabular shells (ie, 58 mm and above), I use larger diameter femoral heads. I would also use them in patients with dementia regardless of shell diameter.

The flouroscopic view (left) shows the insertion of the acetabulum component. The reamer is seated in the acetabulum while reaming (center). During reaming, the cutout reamer appears hemispherical. The acetabular component (right) is in the final placement. COURTESY OF RICHARD A. BERGER

Schmalzried: What is the role of total hip resurfacing with alternate bearings?

Callaghan: If total hip resurfacing does have promise it will be with alternative bearings because the big problem previously has been the acetabular side of the construct. In the past, the acetabulum required excessive reaming, and the polyethylene had to be relatively thin in order to mate the acetabular component with the femoral component. With alternative bearings, especially metal-metal, the thickness of the acetabular shell is no longer the same concern. In addition, with the large femoral heads, impingement and dislocation are less likely to happen.

I am still concerned, however, with the other issues of hip resurfacing which include the ability to reconstruct hip offset and leg length in cases with severe femoral head deformity and the other potential problems with exposure of the joint (potential for femoral nerve palsy and heterotopic ossification). The potential for femoral neck fracture or femoral component loosening remains a concern as this component still needs to be cemented, and sometimes it needs to be cemented onto a bony bed that is not optimal for cement bone interdigitation.

Schmalzried: What is the current role of computer-assisted surgery in THR? What is the potential?

Berger: Computer-assisted surgery has great potential in many aspects of orthopedics, especially arthroplasty. In the future, this technology will be able to assist all surgeons whether they perform a traditional or minimally invasive procedure. Unfortunately, this is not yet at the point of being “user friendly” for most of us; it is expensive, time consuming and usually requires a large exposure.

When it becomes both simple and inexpensive and can be used with minimally invasive techniques, the most significant objection to minimally invasive surgery — possible component malposition — will be minimized.

Berry: The role of computer-assisted surgery in THR continues to evolve rapidly. Several different methods of computer-assisted surgery provide valuable intraoperative information on implant position and leg length. The main problems with computer-assisted surgery have been the expense of the systems and the “fiddle factor” related to using them. To become useful for the everyday practice of most orthopedic surgeons, the cost of these systems must drop and the ease of use must improve dramatically. Both of these improvements, however, can be expected in the future, and if the techniques become simple enough to use, one could envision a time when the technology becomes a routine part of hip arthroplasty.

Schmalzried: Does the current Medicare reimbursement for THR fairly value the cognitive, technical, work effort and risks of the procedure?

Berry: Fair value is a hard number to tabulate, but compared to many other procedures, hip replacement seems markedly underreimbursed and revision hip replacement seems even more markedly underreimbursed. The reimbursement rate for hip replacement, which has been reduced dramatically in the last decade, is probably not in concert with the technical and cognitive efforts required, as well as the amount of work and risk involved in performing this procedure and caring for these patients perioperatively.

Maloney: There is no question that hip replacement is underreimbursed by Medicare.

Reimbursement for hip and knee replacement has decreased about 50% over the past 15 years. This is not because the procedure is any less technical or being done on healthier patients. Quite the contrary: The technical complexity associated with performing joint replacement surgery has increased, patients’ expectations have increased, and we are operating on sicker patients.