Total Knee Arthroplasty (TKA), commonly known as a total knee replacement, is an orthopedic procedure performed to alleviate pain and restore function in knees severely damaged by arthritis. The procedure involves resurfacing the ends of the femur and tibia with metal components and inserting a plastic spacer between them to recreate a smooth joint surface. A complete dislocation of the artificial joint can occur, but it is an extremely rare event following a primary TKA. This complication has a very low incidence, typically reported to be less than 1% of all cases.
Defining Dislocation in Total Knee Replacement
Dislocation represents the most severe form of instability that can affect a replaced knee joint. In the context of a TKA, dislocation means the articulating surfaces of the implant have separated, which is distinct from a dislocation of the natural knee joint involving torn ligaments. This typically involves the polyethylene insert, the plastic spacer that functions as the artificial cartilage, slipping entirely out of its proper position relative to the metal femoral and tibial components. The separation of the femur and tibia from the tibial component is known as a femorotibial dislocation. This complication is uncommon because the implant design, along with the surrounding soft tissues, is engineered to provide inherent stability. For modern implant designs, the prevalence rate for complete dislocation after primary TKA generally falls in the range of 0.15% to 0.5% of cases.
Specific Causes of Implant Instability
The factors contributing to implant instability can be grouped into mechanical and patient-related categories.
Mechanical and Surgical Factors
Mechanical and surgical errors are frequent causes, often traced back to issues during the initial operation. Component malalignment, such as the incorrect rotation or positioning of the tibial or femoral component, can place undue stress on the joint mechanisms. An imbalance in the surrounding soft tissues, particularly the ligaments, is another common surgical factor leading to instability. If the ligaments are released too extensively to correct a pre-existing deformity, the knee may be left too loose, creating a mismatch between the flexion and extension gaps. Inadequate sizing of the prosthetic components can also contribute to this problem, such as using an undersized femoral component that results in excessive laxity in the joint.
Patient-Specific Factors
Patient-specific factors also increase the risk of instability. These include severe pre-existing knee deformities, such as a major varus or valgus alignment, which require a large surgical correction that can compromise soft tissue integrity. Patients with a high body mass index (obesity) exert greater mechanical stress on the implant, which can increase the risk of component separation. Furthermore, patients with certain neuromuscular disorders or who experience high-energy trauma, such as a severe fall, may be more susceptible to dislocation.
Recognizing the Signs of Dislocation
A dislocated knee replacement presents with immediate symptoms that require urgent medical attention. The patient typically experiences the sudden onset of severe, sharp pain, often accompanied by a distinct feeling of the knee “giving out.” This pain makes it impossible to bear weight or move the affected limb normally. A visible deformity or unnatural appearance of the knee joint often accompanies the pain, as the components are no longer correctly aligned. The knee may appear swollen quickly due to joint effusion, and the patient may describe a complete loss of stability. These sudden and severe symptoms should be treated as a medical emergency because displaced components can potentially compress or damage nearby nerves and blood vessels.
Treatment and Recurrence Prevention
Initial Treatment
The initial medical intervention for a dislocated TKA is an urgent procedure known as a closed reduction. This involves manually manipulating the leg to reposition the implant components back into their correct anatomical alignment, usually performed under sedation or general anesthesia. Following a successful reduction, X-rays are taken immediately to confirm proper alignment. A thorough assessment of the limb’s neurovascular status is also performed to check for any damage to the arteries or nerves.
Preventing Recurrence
After the acute event, the focus shifts to preventing recurrence, often involving a long-term strategy. Physical therapy is prescribed to strengthen the muscles surrounding the knee, which helps provide dynamic stability to the joint. If the dislocation was an isolated event with no underlying mechanical cause, this conservative approach may be sufficient. If the dislocation is recurrent or linked to technical errors like component malalignment or persistent ligamentous laxity, revision surgery is necessary. During revision, the surgeon corrects the alignment or rebalances the soft tissues. Often, the existing implant is replaced with a more constrained design, such as a posterior-stabilized or a rotating-hinge prosthesis, to provide greater mechanical stability and prevent future separation.