In one-third of one percent (0.32%) of the total hip patients in this study from Thomas Jefferson University Hospital in Philadelphia, Pennsylvania, a serious nerve problem developed. The peroneal nerve (a branch of the much larger sciatic nerve) was pinched, pressed, cut, deprived of oxygen, or exposed to high heat. The result was muscle weakness, loss of normal motor function (movement), and altered sensation.
The large sciatic nerve splits just above the knee to form the tibial nerve and the common peroneal nerve. These two nerves travel to the lower leg and foot, supplying sensation and muscle control. The tibial nerve continues down the back of the leg while the common peroneal nerve travels around the outside of the knee and down the front of the leg to the foot. Either one of these nerves can be damaged by injuries around the knee.
Although this problem only affected a very small portion of the entire patient group (31 of the 9,570 patients), the effects were devastating. Studies like this one may help identify the cause(s) of this rare, but serious complication. The hope is to use this information to help keep them from affecting others in the future.
By taking a look at all the patient characteristics of the 31 patients and comparing them to the rest of the (much larger) group, the authors were able to see one risk factor predicting peroneal nerve palsy. Younger age (less than 56 years old) increased the risk of nerve palsy and poor or slow recovery.
They also saw one prognostic factor (i.e., able to predict when peroneal nerve palsy is more likely to happen). Being significantly overweight (obese) influences recovery in a negative way. In other words, a larger body-mass index (BMI) is a red flag for potential nerve problems associated with total hip replacement.
Some of the patients (40 per cent) who developed nerve palsy showed signs of this emerging problem in the first day after surgery. Most of the rest of the group were diagnosed on the second day after surgery. A few others developed significant nerve palsy sometime during that first week after the procedure (from day three up to day seven).
Despite CT scans and MRIs, the exact cause of the peroneal palsy was not always known. Some of the reasons why patients developed this type of nerve palsy included compression from a hematoma (pocket of blood), lipoma (benign fat tumor), and screw used in the hip replacement (pressing against the nerve).
There were quite a few cases from traction (pulling) on the nerve either directly or indirectly from the use of a surgical tool known as a retractor. In a smaller number of patients, hip dislocation or limb lengthening contributed to nerve damage leading to nerve palsy. And one patient developed nerve palsy from being in a position that put pressure on the lower leg bone (fibula), which in turn, pressed against the nerve.
Time to recovery ranged from one month up to 50 months (four years, two months). Time in the operating room and amount of blood loss did not seem to be significant factors in recovery time. Body size/weight was the main determining factor in whether or not there was full recovery (not whether it was complete or incomplete at the time of the injury).
This study was unable to identify a single body mass index (BMI) threshold (the number at which a nerve palsy is likely to develop). There was a trend observed: the higher the BMI, the greater the chance for incomplete recovery. The lower the BMI, the more likely the patient was to experience full sensory and motor recovery.
In the end, only slightly more than half (57 per cent) of the patients with a peroneal nerve palsy recovered fully after their total hip replacement. In general, it seems to take about a year for patients to regain as much of the lost sensory and/or motor function as possible. In some cases, this time period was extended to 18 months (one and a half years). Preventing this problem may depend on weight loss before surgery. Other preventive measures involve avoiding pressure on or stretch of the nerve from surgical technique or patient positioning.