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Mid-term results show approach, large THA heads may protect against early dislocation
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CHICAGO — Surgeons who increased their use of large 36-mm and 40-mm diameter femoral heads in the last 10 years after highly crosslinked polyethylene was introduced reported that this change in practice helped them attain lower primary total hip arthroplasty dislocation rates.
Kevin I. Perry, MD, presented an update on a previous study his group did of dislocation after primary total hip arthroplasty (THA) at the American Academy of Orthopaedic Surgeons Annual Meeting. In their initial study, they found posterior approach and small-sized femoral heads were associated with increased rates of dislocation.
Discussing the latest findings, Perry said, “With [these] data, we see that the larger the head diameter, the lower the dislocation rate. Nevertheless, we are using these larger femoral heads judiciously using primarily 32 mm and 36 mm for a routine primary total hip. We are not advocating using 40-mm femoral head diameters for routine primaries due to risks of trunnionosis and corrosion, and unknown long-term wear rates.”
Images: Perry KI
Drop in dislocation rates
The results Perry presented included 5-year dislocation rates of 0.7% with 36-mm diameter heads implanted with an anterolateral approach and rates of 2% with a posterior approach to implant the same size femoral heads.
“The take-home points from this study are that: larger femoral head diameters are associated with a lower cumulative risk of dislocation; that this effect seems to be protective on the early dislocations, but, it is unclear at this point what will happen in the long-term; and lastly, though again seen for all operative approaches, this protective effect continues to be greatest when using the posterolateral approach,” Perry said.
He and colleagues studied 31,119 primary THAs performed at the Mayo Clinic, in Rochester, Minn., USA, from 1969 to 2010 with femoral heads sizes from 22 mm to 40 mm in diameter. Of those, 10,000 THAs were completed in the interceding years since the group first studied THA dislocation, Perry said.
“We defined dislocation [in both series] as an event requiring a reduction by a physician. No subluxation events were included,” Perry said.
Surgeons performed most of these THAs with an anterolateral approach (13,739 procedures). About 9,000 THAs each were performed with a posterolateral or a transtrochanteric approach. Most of the transtrochanteric approaches were done in the first series.
“Since the time of our previous series, the posterolateral [approach] has eclipsed the anterolateral approach in terms of its overall frequency of use,” Perry said, noting the use of 36-mm and larger diameter femoral heads at the Mayo Clinic steadily increased in the last decade.
Perry said, “The cumulative probability of a first time dislocation is affected by the type of approach used. As we are all aware, the posterolateral approach tends to be associated with higher dislocation rates. Nevertheless, this effect is stratified for in our analysis.”
Cumulative dislocation risk
In all, 1,059 dislocations occurred during the study period for a 1.9% cumulative risk of first THA dislocation at 1-year follow-up. This represented a decrease in risk of dislocation from the first study, according to the abstract, which noted the risk of first dislocation at 5 years, 10 years, 20 years and 30 years postoperatively was 2.5%, 2.9%, 3.3% and 3.4%, respectively.
The multivariate analysis that Perry and colleagues performed with their updated research compared heads sized 22-mm through 36-mm diameter. It showed a statistically significant steady decrease in relative risk (RR) of dislocation as the diameter of the femoral head they used increased, with 0.32 RR for 36-mm heads and 0.08 RR for 40-mm and larger heads.
One study weakness Perry mentioned was surgeons did specialized posterior capsular repairs after THA using the posterolateral approach, but were not performing this formal capsular repair in their previous series. – by Susan M. Rapp
- Reference:
- Perry KI. Paper #32. Presented at: American Academy of Orthopaedic Surgeons Annual Meeting; March 19-23, 2013. Chicago.
- For more information:
- Kevin I. Perry, MD, can be reached at Mayo Clinic, 200 1st St. SW, Rochester, MN 55905, USA; email: perry.kevin@mayo.edu.
Disclosure: The two senior authors receive royalties from Biomet and DePuy. The investigators also receive institutional support from multiple companies.