Surgeon volume and early complications after primary total hip arthroplasty

Within the past 50 years, total hip arthroplasty has developed from a procedure with a low success rate to one of the main achievements in surgery and has even been designated the operation of the century.1 The tipping point came during the 1960s, when Sir John Charnley designed and developed a unique well functioning prosthesis and used acrylic bone cement to fix the components. Early (already good) results with this prosthesis have been further improved by later devices. Clinical outcomes are now excellent, and over 95% of modern hip prostheses survive for at least 10 years. Complications do occur, however, and include instability, aseptic loosening, periprosthetic fracture, infection, and occasionally death. Although rare, complications do not happen completely at random, and various patient, surgical, and prosthetic factors have been associated with higher risk.

The linked Canadian register study (doi:10.1136/bmj.g3284) further explores the role of surgeon volume—the number of procedures conducted annually by individual surgeons.2 More specifically, the authors were looking for a volume threshold above which risk of early complications was low for adults having a first total hip replacement for osteoarthritis—the main indication for this type of surgery. Total hip arthroplasty is also used for the treatment of osteonecrosis, hip dysplasia, and inflammatory arthritis and increasingly as a primary or secondary treatment for fractures of the femoral neck. Each year more than a million procedures are undertaken worldwide, and avoidance of even a small proportion of complications would substantially reduce both individual suffering and healthcare costs.3

The operation is technically demanding with a long learning curve.3 4 The importance of high surgeon volume in preserving technical skills has previously been reported by several earlier register studies.5 6 Besides a generally careful analysis, the main advantage and novelty in the study by Ravi and colleagues is their use of restricted cubic spline curves to visualise the inflection point and therefore the threshold in surgeon volume associated with higher risk of complications.2 The method is well established, although this is the first attempt to use it in this particular clinical research topic.7 8 A standard quantile analytical approach comparing a few categories, such as fourths, has well described shortcomings and would not have shown the true pattern of associations reported here between surgeon volume and complication rates.7 The cut offs chosen with a quantile approach are often selected arbitrarily and do not reflect risk limits. Furthermore, risks can vary substantially within quantile ranges.8 The Canadian researchers concluded that at least 35 procedures a year or three a month are needed to reduce the frequency of dislocations or revisions within the first two years after surgery. Patients treated by surgeons above this threshold had roughly 30% risk reduction of early dislocations or revisions compared with patients treated by surgeons below the threshold.

In absolute terms, the analysis suggests that one dislocation or revision would be prevented in every 100 or so patients switched from a low volume to a high volume surgeon. It’s important to note that for patients treated below the threshold (35 procedures a year), risk of complications continued to rise steadily as surgeon volume fell. The results remained robust through several sensitivity analyses, providing reassurance of validity of the findings.

Restriction of the study to patients with osteoarthritis is a further asset of the design as the inclusion of additional treatment indications might have distorted the estimates. Interestingly, the results were independent of surgeon experience in years. Surgeon volume was not associated with mortality rates or risk of infection, periprosthetic fracture, or venous thromboembolism—perhaps because some of these outcomes are more readily explained by intrinsic patient factors than by the surgeon’s performance.

Innate surgical skill that is unrelated to experience or yearly volume is an important confounding factor in these associations and one that Ravi and colleagues were unable to control for. Less skilled surgeons (with potentially higher complication rates) might voluntarily or involuntarily choose to perform fewer procedures, focusing instead on administration, teaching duties, outpatient visits, or even research. The link between innate surgical skill and surgical complications is a delicate issue but has recently been shown in a study in surgeons performing bariatric surgery.9 Those judged on video by their peers as talented surgeons, without personal knowledge of that surgeon, also had fewer complications.9 Surgeons themselves know who they would like to have as their own surgeon.

Ravi and colleagues’ study has some additional possible limitations including a lack of information on prosthesis type and surgical approach as well as patients’ anthropometry and smoking habits.2

Early complications that occur within two years of a total hip replacement constitute a minority of all complications, and the generalisability of these findings to longer term outcomes is uncertain. Late onset aseptic loosening of the prosthesis is a major problem, a complication not dealt with in the present investigation.2 Previous research suggests that patients treated by low volume surgeons are also at higher risk of complications occurring several years after the procedure.10

Most previous research on total hip arthroplasty has been devoted to the development of the design, prosthesis type, and fixation method, and high quality register data have been extremely important in the identification and removal from use of poorly performing devices and bone cement products.11 12 Few studies have explored the impact of surgeon related factors and skills. The thorough analysis by Ravi and colleagues is one of the best attempts so far. Should low volume orthopaedic surgeons stop performing surgery or try to increase their annual number of operations? It’s hard to say at this stage. More equally rigorous studies are required. In the meantime, surgeons would be well advised to keep their engines running at fairly high speed.