A total knee arthroplasty (TKA) replaces a damaged knee joint to relieve chronic pain and restore function. While generalized discomfort is expected during recovery, new or localized pain on the outer side of the knee can be concerning. This specific lateral pain often signals an issue distinct from typical post-operative recovery, such as irritation of soft tissues, a nerve, or a mechanical issue related to the new implant. Understanding these underlying causes is the first step toward effective treatment.
Iliotibial Band Friction
The most frequent cause of isolated pain on the outside of the knee following TKA is irritation of the iliotibial (IT) band. This thick, fibrous band runs from the hip down the outside of the thigh and normally glides smoothly over the lateral femoral condyle (the outer bony prominence).
Following TKA, the IT band can become inflamed, resulting in iliotibial band friction syndrome (ITBFS). Surgical trauma and swelling can alter knee mechanics, causing the band to rub excessively against the implant or surrounding structures. This friction is often exacerbated by changes in gait and alignment as the patient increases activity during physical therapy.
ITBFS symptoms are typically a sharp or burning pain felt directly on the lateral knee. The pain is often worse when the knee is bent or straightened repeatedly, such as when walking or climbing stairs. Occasionally, the pain is caused by an obstruction, like extruded cement or an unresected piece of bone, which creates a prominent point rubbing against the band.
Initial treatment focuses on non-surgical methods, including rest, applying ice, and specific physical therapy exercises. Rehabilitation primarily focuses on stretching the hip abductor muscles and the IT band to restore flexibility and reduce tension. If conservative treatments fail and a mechanical obstruction is identified, a minor surgical procedure may be considered to excise the prominence and relieve persistent friction.
Peroneal Nerve Irritation
A less common source of lateral pain is irritation of the common peroneal nerve. This nerve is a branch of the sciatic nerve that wraps closely around the fibular head, the bony knob just below the knee. Due to its superficial location, the nerve is vulnerable to stretching or compression during the knee replacement procedure.
The nerve can be stretched when the surgeon corrects a pre-existing severe angular deformity, such as a valgus (knock-kneed) alignment. Post-operative swelling or hematoma formation can also compress the nerve against the bone. Unlike IT band friction, nerve irritation presents with distinct neurological symptoms.
Patients may experience shooting pain, numbness, or tingling (paresthesia) along the outer side of the lower leg and the top of the foot. The most concerning symptom is motor weakness, known as foot drop, which is the inability to lift the front part of the foot while walking. The incidence of this complication is relatively low.
If nerve symptoms are suspected, immediate monitoring is necessary. Management may involve adjusting the knee position or observing for spontaneous recovery, as many cases resolve over time. If weakness is severe and persistent, or if sensory symptoms interfere with daily life, a nerve decompression procedure may be discussed.
Mechanical Alignment and Implant Position
Lateral knee pain can be directly related to the positioning or sizing of the prosthetic components. TKA success relies on restoring the mechanical axis of the limb, the straight line of weight-bearing from the hip to the ankle. Deviation from optimal alignment, such as slight malrotation of the components, alters the tension on surrounding soft tissues.
Component malrotation creates an imbalance in the ligaments and tendons, leading to chronic irritation and lateral pain. Additionally, if the tibial or femoral component is slightly oversized or malpositioned, it can cause a lateral overhang. This overhang creates an abnormal point of contact that physically rubs and irritates adjacent soft tissues, including the iliotibial band attachments.
These mechanical issues are evaluated using specialized imaging, such as long-leg X-rays to assess limb alignment or CT scans to measure rotational positioning. If the malalignment is minor, targeted physical therapy can strengthen muscles to compensate. However, if pain is severe and persistent, and imaging confirms significant malpositioning or component overhang, revision surgery may be necessary to correct the mechanical issue.
Red Flags for Serious Complications
While most causes of outer knee pain post-TKA are manageable, certain symptoms signal a serious complication requiring immediate medical attention. The most concerning signs relate to periprosthetic joint infection (PJI).
Signs of PJI include:
- Persistent or worsening fever and chills.
- Severe pain that does not respond to medication.
- A noticeable increase in swelling and redness around the knee.
- New drainage from the incision, especially pus or cloudy fluid.
Other red flags relate to implant stability and integrity. These include the sudden onset of a loud popping or clicking sound followed by a feeling of instability or the knee giving way. Immediate inability to straighten or lift the leg (extensor mechanism disruption) is also an acute complication requiring urgent intervention. If these systemic or acute mechanical symptoms appear, prompt communication with the orthopedic surgeon is necessary.