A total knee replacement (TKR) involves replacing the worn-out surfaces of the knee joint with artificial components. These components typically include a metal cap on the thigh bone (femur), a metal tray on the shin bone (tibia), and a plastic spacer that acts as the new cartilage. While the goal is to restore mobility and eliminate pain, a common concern is what happens if the knee sustains a sudden impact, such as from a fall. A fall can cause severe damage to the knee system, but it is extremely rare for the implant components themselves to break. The real vulnerability lies not in the artificial parts but in the surrounding bone structure that holds the implant in place.
Understanding the Risk: Implant Integrity vs. Bone Integrity
Modern knee replacements are engineered using highly durable materials, such as cobalt-chromium and titanium. These metals possess exceptional strength and resistance to mechanical failure, meaning they can withstand significant force before fracturing. The polyethylene plastic spacer, while considered the “weak link” for long-term wear, is also designed to be highly resilient against acute trauma.
The components are secured to the bone either using specialized bone cement or through biological fixation, where the bone is encouraged to grow directly onto the implant surface. The bone itself near the implant edges becomes the most susceptible point of failure under excessive stress. This failure often occurs because the bone is already compromised, either due to the surgical cuts made to fit the implant or from pre-existing conditions.
Bone quality is the primary factor influencing the risk of injury from a fall, with conditions like osteoporosis significantly weakening the structure. Older age, especially in women, is a known risk factor, as is having inflammatory conditions like rheumatoid arthritis. The intensity and specific angle of the impact force during the fall determine where the stress is concentrated, leading to a fracture in the weaker bone near the metal components.
Recognizing a Periprosthetic Fracture
When a fall causes the bone around the knee replacement to break, the resulting injury is specifically termed a periprosthetic fracture. This type of fracture is a serious complication that most frequently occurs in the thigh bone (femur) just above the metal cap. The shin bone (tibia) or kneecap (patella) can also be involved. Recognizing the signs of this injury is important because timely medical intervention is necessary.
Sudden onset of intense pain localized around the knee joint is a telling symptom, usually following the trauma of the fall. This severe pain is quickly accompanied by a pronounced inability to bear any weight on the injured leg. Patients may also notice significant swelling and bruising in the area surrounding the knee and thigh due to internal bleeding and inflammation.
In more severe instances, the leg may appear visibly shortened or unnaturally deformed, which indicates a displacement of the fractured bone segment. If these symptoms are present after a fall, seek emergency medical attention immediately. Diagnosis typically begins with simple X-rays to visualize the fracture location and the stability of the implant.
In some cases, a Computed Tomography (CT) scan may be utilized to provide more detailed information about the extent of the bone damage. This assists surgeons in planning the appropriate repair. A comprehensive evaluation will also assess the overall condition of the patient and determine if the existing implant has become loose due to the fracture.
Surgical Repair and Post-Injury Recovery
Treatment for a periprosthetic fracture depends on the fracture’s location, the overall quality of the surrounding bone, and whether the knee replacement components are stable. Undisplaced fractures with a stable implant may sometimes be managed without surgery using immobilization, such as a cast or brace, along with a period of non-weight bearing. However, the majority of displaced or unstable periprosthetic fractures require surgical intervention.
Open Reduction and Internal Fixation (ORIF)
Open reduction and internal fixation (ORIF) involves realigning the broken bone fragments and securing them with metal plates, screws, or rods. Locking plates are frequently used in these procedures to provide enhanced stability to the compromised bone around the implant. This method is favored when the existing knee replacement remains firmly fixed in the bone.
Revision Surgery
If the fracture has caused the original implant to loosen or if the bone stock is severely damaged, a more complex revision surgery is necessary. This involves removing the original components and replacing them with a new, often larger, stemmed implant that extends further into the bone for greater stability. In rare cases involving extensive bone loss, surgeons may need to use a specialized megaprosthesis to replace the entire lower end of the femur.
Recovery from a periprosthetic fracture is prolonged compared to the initial knee replacement surgery. Patients typically face an extended period of restricted weight-bearing, which can last several weeks. Patients require three to six months to fully recover from the surgical repair and regain function. A dedicated course of post-operative physical therapy is essential to help restore strength and range of motion following the procedure.