PLEASE NOTE:
To the authors’ knowledge, interpositional spacers are no longer available in the US.  This is primarily the result of unpredictable performance of the device and poor sales as a result”.

The hallmark of knee degeneration is “joint space narrowing,” which is the Xray manifestation of cartilage wear. A knee Xray will show a diminished space between the bones indicative of degeneration. Various spacers have been introduced to widen the joint space. As a joint space narrows, the leg alignment changes for the worse, and the stress concentration upon damaged cartilage further increases. In theory, a spacer will widen the joint space and shift the load to a more even distribution across the joint surface. Though this concept has merit, success of this procedure has been variable, possibly due to material and size limitations.

Previous iterations of this device have been composed of metal alloys.

Future products will be composed of biocompatible materials, but are not yet FDA approved for distribution and implantation. 

http://www.jbjs.org/Comments/2005/cp_aug05_springer.shtml

Marmor popularized a metallic spacer concept in the 1970’s. Though the procedure demonstrated good results in some patients, it generally lost favor given its unpredictability. The next generation of interpositional spacer surfaced in the 1990’s with the advent of the “Unispacer.” This too was a metallic spacer. It is a C-shaped metal component, which cups the femoral condyle and slides freely on the tibial surface. With extension and flexion of the knee, the Unispacer will migrate forward and backward which supposedly allows for normal joint kinematics. Somewhat similar to the Marmor experience, this procedure also yielded unpredictable results, possibly due to sizing issues. The sizing issue was resolved with the newest generation spacer, called the “iFormis.” This spacer is custom manufactured according to the measurements derived from a preoperative CT scan of the patient’s knee. This is a metallic device that conforms very accurately to the contour of the patient’s femur and tibia.

Though the spacer concept has consistently shown that a subset of patients can do well with this procedure, the fundamental flaw is metal on bone, and worn-out bone at that. Newer materials with properties more similar to cartilage are presently being tested for compatibility and wear characteristics.

The procedure is indicated in those knees where there is substantial joint space narrowing in one compartment of the knee. The other compartments should be normal. Complete bone on bone Xray changes are unfavorable as an indication. Additional contraindications include severe axial malalignment, ligamentous instability and patients with systemic inflammatory disease.

The iFormis is a patient-specific device meant to closely match the contours of a given patient’s anatomy. A pre-operative CT scan is performed of the knee according to specific parameters identified by the manufacturer. The metal component is then custom manufactured according to these specifications. The component is delivered to the surgical department.

An arthroscopic examination of the knee is generally undertaken as an initial step. The knee is examined for loose bodies, meniscal damage and other surface involvement. An incision is made several inches long. Impinging osteophytes are removed. The device is lnserted by levering it into its position within the knee. Range of motion examination is performed to ensure proper placement and articulation.

http://www.righthealth.com/topic/Unispacer_Knee

An overnight stay in the hospital is common. Immediate weight-bearing and range of motion is allowed. The patient can expect to walk with discomfort for the first 6 weeks. Comfortable ambulation may take up to 3 months post-op.

Prior to the advent of the customized spacer, results were inconsistent. However a subset of patients has consistently done well with this device. A major reason for past failures has been due sizing inaccuracies. In early reports, the iFormis customized spacer seems to have better results than its predecessor devices, however the company has questioned its financial viability and discontinued the product line.

References:

1. Bailie, et al. The Unispacer Knee Implant: Early Clinical Results JBJS Br 2008;90-B:446-50.
2. Hallock, RH. The Unispacer Knee System: Have we Been There Before? Orthopedics 2003; 26-9: 953-954.
3. Hallock, RH et al. Unicompartmental Tibial Hemiarthroplasty: Early Results of the UniSpacer Knee. Clin Orthop Rel Res 2003; 418: 154-163
4. Sisto, DJ. et al. Unispacer Arthroplasty of the Knee. JBJS 2005; 87: 1706-1711.
5. Koeck, FX, et al. Prospective Single-Arm Trial of a Patient Specific Interpositional Knee Implant: Early Clinical Results. The Open Orthopedics Journal 1010; 299-305. (this version is ahead of publication)
6. Koeck, FX, et al. Leg axis correction with ConforMIS iForma™ (interpositional device) in unicompartmental arthritis of the knee. International Orthopedics 2008.
7. Koeck, FX et al. Removal and Successful Re-implantation of a customized interpositional knee device. Cent. Eur. J. Med. 20009.