Skin Numbness After Knee Replacement Surgery

Knee replacement surgery is a common surgical procedure that has a very high success rate. While the vast majority of people who have this surgery are very pleased with their results, there are a few reasons why people may not be completely satisfied with their result. The most common problems that can occur after knee replacement include persistent pain, stiffness of the knee joint, and problems with the knee replacement implant. Some of these possible complications can be very serious and even require additional surgical procedures. One of the less serious, and also less common, problems is numbness of the skin around the knee replacement incision.

Sensation of the skin is detected by small nerves that are branches of larger nerves that connects the brain to all of the parts of our body. Sensory nerves are able to detect sensations ranging from pressure, light touch, vibration, and other sensory findings. These nerves form a branching pattern much like the structure of a tree where the major nerves are the trunk, and the sensory nerves to the skin are the fine branches. Damage at any level of this connected pathway can lead to areas of abnormal sensation in the body.

Knee replacement incisions can vary slightly based on surgeon preference, surgical approach, and the implants used. A traditional knee replacement incision is placed directly over the front of the knee. This incision, called a midline incision, is the most common surgical approach for performing total knee replacement.

Other surgical incision options include a medial parapatellar incision and a subvastus oblique incision, both of which are oriented more towards the inner (medial) side of the knee joint. In these situations, a surgeon will shift the incision slightly so that the scar is not directly over the front of the kneecap. The length of the surgical incision can vary dramatically. In a recent study, investigators found the average length of a knee replacement incision to be just over 11 cm, but this varied from about 7 cm up to a length of about 19 cm.

Your body has small, delicate skin sensation nerves that lie directly underneath the skin and cross the area of a knee replacement incision. There is essentially no way to completely avoid the skin nerves when performing knee replacement surgery. When your surgeon makes the initial incision and dissects down to the knee joint, those skin nerves are invariably severed. Even if it were possible to protect the skin nerves at the time of the initial incision, the practicality of preserving them while trying to perform the steps necessary to prepare and implant an artificial knee would be challenging.

As stated, sensory skin nerves are invariably cut at the time of knee replacement surgery. About 50% of people notice symptoms of numbness around the area of the skin incision one year after their surgical procedure. That said, less than 10% of these people are bothered by this symptom.

The three factors which are most notably associated with being bothered by numbness are:

• Length of the Incision: The strongest association with numbness was the length of the surgical incision. Longer surgical incisions are more likely to cut nerves underneath the skin.
• Female Gender: Women seemed to be more commonly affected by abnormal sensations around her incisions, and more than three-quarters of the patients to report discomfort as a result were women.
• Age of the Patient: People older than 70 years of age had fewer complaints about being bothered by abnormal sensations around their incisions.

In addition to these factors, the nerve that provides sensation to the front of the knee starts on the inner side of the joint and progress over the front of the joint extending towards the outer side of the knee joint. When the skin incision is based more towards the inner side of the knee joint, it is more likely to cut the larger portion of the nerve whereas incisions based more towards the outer side of the joint tend to involve the smaller branches. When only the smaller branches are cut, the effect on the perception of sensation is reduced.

Regarding the difference noted in the perception of numbness between men and women, the reason is unclear. Studies have not shown any gender difference of orientation of the skin nerves. Some of this difference may be attributable to differences in clothing. Women specifically mentioned being bothered when wearing skirts or dresses that tended to rub against their incision.

There is not a lot that can be done about numbness around an incision after a knee replacement. The sensations may diminish over time, and there may be some recovery of sensation in this area. Most people who noticed symptoms of numbness find that the area gradually shrinks down over time.

On a positive note, there has never been any evidence that this numbness leads to actual functional limitations on the knee replacement. Even if people are bothered by the abnormal sensation, the knee replacement should function just as well whether or not there is an area of skin numbness.

Damage to the nerves that provide sensation of the skin around the knee joint is common after knee replacement surgery. This nerve damage is seldom noticed by the patient, but some people do have bothersome symptoms as a result. On a positive note, seldom does having abnormal sensation around a knee replacement incision lead to limitations in the function of the knee joint. In addition, abnormal sensations may continue to improve for up to a year from the time of surgery. Limiting the length of the surgical incision is probably the most helpful way to limit the potential damage to sensory nerves.