A total knee arthroplasty (TKA), commonly known as a knee replacement, can dislocate, but this complication is considered very rare. A TKA involves replacing the ends of the thigh bone (femur) and shin bone (tibia) with artificial components, creating a functional joint. The incidence of dislocation in modern primary knee replacements is low, typically ranging from 0.15% to 0.5%. This rate is significantly lower than the dislocation risk associated with total hip replacements, which rely more on soft tissue tension for stability.
Why Knee Replacement Dislocation is Uncommon
The knee joint is naturally a hinge joint, which is inherently stable due to its bony structure and the strong collateral and cruciate ligaments surrounding it. Unlike the hip, the knee’s structure primarily limits movement to a single plane—flexion and extension. This difference in design makes the knee replacement less prone to coming out of place.
When a dislocation does occur, it is often due to a combination of factors, including issues related to the surgical technique or a high-force event. Component malalignment is a major contributing cause, leading to an imbalance in the flexion and extension gaps and making the joint unstable.
Instability can also arise from soft tissue issues, such as ligament deficiency following surgical release for a severe deformity. Patient-related factors, such as severe preoperative deformity, obesity, or neuromuscular disorders, can also increase the risk. In complex cases, surgeons may use specialized implants like constrained or hinged components, which have a built-in mechanism to prevent the joint from separating.
Recognizing the Immediate Signs of Dislocation
A dislocated knee replacement requires immediate medical attention. The patient will experience sudden, excruciating pain, which is far more intense than routine post-operative discomfort. This pain is accompanied by a complete inability to bear any weight on the affected leg.
The knee joint may also exhibit a visible deformity. Often, the patient will report a sensation of the joint “giving way” or feeling completely unstable at the time of the event. Any suspicion of dislocation must be treated as an orthopedic emergency to prevent potential damage to surrounding nerves and blood vessels.
Steps for Preventing Dislocation After Surgery
Preventing a dislocation requires strengthening the surrounding support structures. Adherence to a structured physical therapy (PT) program is a primary defense, as strengthening the quadriceps, hamstrings, and other surrounding muscles provides dynamic stability to the joint. Strong muscles help maintain the correct tracking of the implant.
Patients should actively avoid high-impact activities, such as running, jumping, or sports that involve sudden stops and starts, especially during the initial recovery phase. Follow the surgeon’s specific range-of-motion restrictions, which often involve avoiding extreme flexion or extension for the first few months. Minimizing the risk of falls is also important, as severe trauma is a major cause of dislocation; this includes modifying the home environment by removing tripping hazards and ensuring adequate lighting.
Medical Management Following a Dislocation
Once a dislocated knee replacement is diagnosed, the first step is typically a closed reduction, where the joint is gently manipulated back into position. This procedure is usually performed under sedation or general anesthesia due to intense pain and muscle guarding. Following a successful reduction, immediate imaging, such as X-rays, is necessary to confirm correct alignment and check for fractures.
A neurovascular exam is performed to ensure the dislocation did not damage the popliteal artery or the peroneal nerve. If the dislocation is a one-time event and the components appear well-positioned, the patient may be managed non-operatively with immobilization and physical therapy. However, if the dislocation is recurrent, or if the event revealed significant ligamentous instability or component malposition, revision surgery is often necessary. This involves replacing components with a more constrained design to restore long-term stability.