Sonoma County Medical Association
Michael Star, MD
Total joint replacement is one of the most successful contemporary surgical procedures. Refined surgical techniques, improved implant characteristics, aggressive physical rehabilitation and a focus on pain management have all contributed to this achievement. The long-term success rates for total hip and total knee surgery currently exceed 95%.[1,2]
About 200,000 total hip replacements and 450,000 total knee replacements are performed in the United States each year. By 2030, according to estimates, those numbers will grow to 600,000 hip replacements and 3 million knee replacements.
At this time, most total hip procedures use non-cemented porous ingrowth femur and acetabular components, where a rough or porous surface allows bone to grow in to secure the metal prosthesis. For osteoporotic patients, however, the femoral stem may need to be securely fixed to bone with a cementing technique. Because of the different skeletal anatomy of the knee, most knee replacements are cemented, regardless of patient type. The most common articulating surface in both hip and knee replacements is a surgical grade cobalt-chromium alloy, rotating or gliding on a specially formed ultra-high-molecular-weight polyethylene surface.
Historically, the ideal patient for total joint arthroplasty was a thin, older individual with a sedentary lifestyle. These patient characteristics created less mechanical stress on the implant and could allow the prosthesis to function for the lifetime of the patient without need for revision. Studies had shown that younger, more active, and heavier patients had a higher failure rate.
Changing expectations have significantly altered the patient demographics in total joint replacement. Younger patients who want to continue their active lifestyle are now requesting the surgery. Likewise, overweight patients and those with comorbidities, who would have been rejected in the past because of high risk, are now expecting relief of their symptoms.
While the long-term results of joint replacement surgery are excellent, the changes in patient demographics mandate further research into optimizing results and diminishing surgical and medical complications. Replacing living human joints with artificial metals and plastics is an ongoing technical challenge.
The most common mode of failure has been polyethylene wear, with wear debris resulting in osteolysis and loosening. However, recent improvements in the manufacturing of polyethylene, including cross-linking and oxygen-free sterilization, have made the material more durable and longer-lasting.
For total hip replacements, metal-on-metal bearings have been used since the 1990s, with significantly increased use in the last decade. While most of these implants have had good results, some have loosened from their bond to bone. These failures led to a highly publicized recall and to the recent decreased use of metal-on-metal implants.
Another bearing surface in total hip replacement, ceramic articulation, has been used for about 40 years. Initial results were suboptimal, although improved manufacturing techniques give these prostheses extremely low wear rates and low rates of osteolysis. Clinical results are good, though some ceramic hip bearings have audible squeaking. The squeaks occur in certain hip positions in walking, squatting or climbing stairs, and they may be related to component positioning, which may also cause increased wear.
Taking all these factors into account, the most commonly used articulation, with the most extensive clinical follow-up for hip and knee arthroplasty, remains metal on contemporary polyethylene.
In addition to new materials, new surgical techniques have been developed in an attempt to improve results for hip and knee arthroplasty. As always, the potential benefits of new techniques and technology need to be weighed against increased risk, operative time and blood loss.
Minimally invasive surgery has the theoretical advantages of less blood loss and quicker recovery. Unfortunately, there is little scientific evidence for improved recovery or clinical result, and some studies have even documented an increased complication rate with minimal-incision techniques. Computer-assisted implant surgery is another alternative, but again studies have failed to show consistently improved radiographic outcomes, and clinical improvement has not been clearly documented.
In light of the above, meticulous surgical practice using accepted techniques and proven implants is still the mainstay of successful arthroplasty surgery. Of course, satisfactory results following total joint arthroplasty depend upon minimizing medical complications. In a database review, the 90-day mortality following total hip replacement was 0.68%. The most common implant-related complication was dislocation, and the most significant medical complications were infection and venous thromboembolism. Patient-related risk factors included diabetes, rheumatoid arthritis, increased age and increased Charlson comorbidity score.
The most recent meeting of the American Association of Hip and Knee Surgeons, which I attended, included extensive discussions of the risks and effects of medical complications. Several papers published in the Proceedings addressed the common difficulty of diagnosing infections in post-surgical total joints. Schwartz et al reiterated the use of erythrocyte sedimentation rate, C-reactive protein and synovial fluid white blood cell count, particularly in knee arthroplasty. Wetters et al found leukocyte esterase reagent strips, commonly used for diagnosis of urinary tract infection, helpful for diagnosing infected joint fluid. Cashman et al also found joint-fluid C-reactive protein helpful in diagnosing infected joints. Toossi et al, however, found no benefit in serum white blood cell count in making the diagnosis of an infected total joint.
Morbid obesity has been associated with poor results in total joint replacement. A survey of hip and knee surgeons found that more than 80% discouraged joint replacements because of obesity. Almost 40% referred a total joint replacement candidate to a bariatric program. Similarly, 45% deferred or canceled surgery because of an elevated hemoglobin A1c, the marker of diabetic control.
The importance of identifying and preoperatively addressing other medical comorbidities was discussed at length. Ong et al emphasized the importance of a preoperative risk stratification to decrease renal, pulmonary, mental and cardiac complications. D’Apuzzo et al demonstrated the association between obstructive sleep apnea with increased mortality and morbidity, including pulmonary embolism and wound problems. They recommend preoperative screening and appropriate monitoring and treatment. Romine et al discussed a risk calculator for a total knee arthroplasty. Factors associated with increased risk included hyperkalemia, bilateral procedures, males and increased age. Restrepo et al found that preoperative anemia was associated with increased hospital stay, infection and mortality. Finally, Berend et al described a three-fold increase in failure rates in hip replacements in smokers.
While the success of joint replacement surgery has been extensively documented, changing patient demographics and a low but defined rate of complications mandate continuing study and investigation to optimize surgical and medical results. The orthopedic community is becoming increasingly aware of the importance of appropriately identifying and managing medical comorbidities.
Dr. Star is an orthopaedic surgeon at Santa Rosa Orthopaedics.
1. Lonner JH, et al, “What’s new in adult reconstructive knee surgery,” J Bone Joint Surg Am, 89:2828-37 (2007).
2. Huo M, et al, “What’s new in total hip arthroplasty,” J Bone Joint Surg Am, 90:2043-55 (2008).
3. Parvizi J, et al, “Bearing surface materials for hip, knee and spinal disc replacement,” Orthopedic Knowledge Update 10 (2011).
4. Woolson S, et al, “Comparison of primary total hip replacement performed with a standard incision or a mini- incision,” J Bone Joint Surg Am, 86:1353-58 (2004).
5. Stulberg S, “Computer-assisted surgery versus manual total knee arthroplasty,” J Bone Joint Surg Am, 88(S4):47-54 (2006).
6. SooHoo N, “Factors that predict short-term complication rate at the total hip arthroplasty,” Clin Ortho Rel Res, 468:2363-71 (2012).
7. American Association of Hip and Knee Surgeons, Proceedings at the 21st Annual Meeting, AAHKS (2011).
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