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Larger thigh girth increases risk for failed patellar eversion in subvastus approach to TKA
A preoperative knee range of motion less than 120° also increases the risk for failed patellar eversion.
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The presence of a thigh girth over 55 cm may be a contraindication to performing primary total knee arthroplasty using the subvastus approach, a retrospective study indicates.
"[We] found that patients with a thigh girth greater than 55 cm had a much higher risk for inability to evert the patella as compared with patients with a small thigh girth. It meant that patellar eversion was seldom possible in patients with a thigh girth greater than 55 cm," the authors wrote in the study.
Yong In, MD, and colleagues at the Catholic University of Korea, Seoul, reviewed factors affecting the ability to evert the patella in a consecutive series of 143 knees in 96 patients. All patients underwent primary total knee arthroplasty via the subvastus approach at a mean age of 67.9 years.
The results are published in the Journal of Arthroplasty.
The researchers found that the patella could be everted and dislocated laterally in 109 of the 143 knees. In the remaining 34 knees, the patella could not be everted and the surgeon had to laterally slide the patella and quadriceps mechanism for exposure, according to the study.
Preoperatively, investigators calculated mean range of motion (ROM) and measured thigh girth at 10 cm above the patella upper pole in all cases.
Initially, four factors were associated with inability to evert the patella — Body Mass Index (BMI), thigh girth, full flexion angle and knee ROM.
Both BMI and thigh girth showed significant correlations with inability to achieve patellar eversion. However, "We believed that thigh girth was a better predictor ... than was BMI because BMI did not necessarily indicate thigh obesity," the authors wrote.
To calculate the risk for failed patellar eversion, investigators grouped knees into four classes based on thigh girth. Class 1 included 85 cases with a thigh girth less than 45 cm. Class 2 included 34 cases with a thigh girth of 45 cm to 49.9 cm. Class 3 included 15 cases with a thigh girth of 50 cm to 54.9 cm, and Class 4 included nine cases with a girth of 55 cm or more, according to the study.
The researchers found that, compared to Class 1 patients, patients in Class 2 had a 3.931-times greater risk for failed patellar eversion. Class 3 patients had a 4.539-times higher risk, and Class 4 patients had a 27.212-times higher risk, according to the study.
Preoperative knee ROM also affected the risk for failed patellar eversion. Although not significant, patients with a with a ROM of 120° to 139° had a 2.284-times higher risk compared to patients with a ROM of 140° or more.
However, also compared to patients with a ROM of 140° or more, those with a ROM of 100° to 119° had an 8.505-times higher risk (P=.013) and those with a ROM less than 100° had a 7.937-times higher risk (P=.001), according to the study.
"The [odds ratio] study showed that the larger the thigh girth, the higher the risk for inability to evert the patella," the authors wrote, noting that ROM less than 120° also significantly increased the risk.
"Knowledge of factors affecting patellar eversion will aid in planning for an approach in TKA," they wrote.
"We can seldom or never evert the patella in patients with a thigh girth greater than 55 cm. We recommend that another approach instead of the subvastus approach be used in these patients," they added.
For more information:
- In Y, Kim JM, Choi NY, Kim SJ. Large thigh girth is a relative contraindication for the subvastus approach in primary total knee arthroplasty. J Arthroplasty. 2007;22:569-573.