Interpositional unicondylar spacers are metallic implants that are inserted into the joint space between the affected tibial plateau and femoral condyle. Instead of being fixed, the spacers are held in place by the geometry of the curved implant, ligament tension, and surrounding soft tissue structures. Metallic interpositional unicondylar spacers have been developed as possible alternatives to osteotomy or unicompartmental arthroplasty for treatment of unicompartmental knee arthritis.
While osteoarthritis of the knee may involve both medial and lateral components, some patients may have signs or symptoms confined to only one compartment, often as a result of specific injury. Initial treatment of uni- and bi-compartmental disease is similar, i.e., analgesics, viscosupplementation, and arthroscopic debridement. If focal cartilage defects are present, other therapies such as autologous chondrocyte or osteochondral transplants may be considered. When these more conservative therapies fail, surgical interventions may be indicated.
Older patients with arthritis who typically consider total knee replacement are the type of patients who might be considered for unicondylar knee arthroplasty. The metal-to-plastic unicondylar prosthesis has both tibial and femoral components but leaves intact both cruciate ligaments, the patellofemoral joint, and the opposite compartment, thus preserving nearly normal knee kinematics. Unicompartmental arthroplasties are typically performed in patients with isolated medial arthritis who are older than 60 years, have a low level of physical activity, and weigh less than 180 pounds.
Younger patients are usually not considered ideal candidates for total knee arthroplasty since they would be expected to need at least one additional total knee replacement in their lifetimes. The preferred option for young patients with unicompartmental disease is high tibial osteotomy. Those with medial compartment disease may also be candidates for unicompartmental arthroplasty or for fixed metallic hemiarthroplasty. MacIntosh and McKeever hemiarthroplasty devices were used primarily between 1950 and 1970, but their use has decreased with the refinement of total knee arthroplasty procedures. These devices require bone cuts that might complicate future arthroplasty procedures.
Metallic interpositional unicondylar spacers have been developed as possible alternatives to osteotomy or unicompartmental arthroplasty. These devices do not require any bone resection or mechanical fixation for proper function. Following debridement and resection of the meniscus, the device is fit to the joint space above the affected tibial plateau and held in place by its geometry, ligament tension and the surrounding soft tissue structures. The uncemented implant adapts to the kinematics of the knee, with a smooth metallic curved surface against which the femur articulates. Preservation of bone is important for the use of interpositional spacers as a bridge procedure in active young adults or for overweight patients who would not be candidates for unicompartmental arthroplasty.
In 2001, a cobalt alloy device, referred to as a "unicondylar interpositional spacer" or "UniSpacer™" received clearance for marketing through a U.S. Food and Drug Administration (FDA) 510(k) process as an interpositional arthroplasty device, listing the MacIntosh and McKeever prostheses as predicate devices. The FDA clearance for the UniSpacer device is for "moderate degeneration of the medial compartment of the knee (grade III-IV chondromalacia) with not more than minimal degeneration (grade I-II chondromalacia, no loss of joint space) in the lateral condyle and patellofemoral compartments." The Orthoglide Medical Knee Implant™, which received 510(k) approval in 2006, is also indicated for “uncemented use in the treatment of moderate degeneration of the medial compartment of the knee,” but for grade II-IV chondromalacia.
Two other interpositional devices have received 510(k) approval, the Oti Unicondular Interpositional Spacer Osteoimplant™ (2002) and the Knee Interpositional Mini-Repair System™ (2003), listing the UniSpacer as a predicate device. The Knee Interpositional Mini-Repair System is a patient-specific design, with specifications taken from magnetic resonance scans. These devices are indicated for the uncemented treatment of medial and/or lateral tibial articulating surfaces of the osteoarthritic knee with grade II-IV chondromalacia.