New instrumentation techniques in mobile bearing UKA optimize surgical results

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Unicompartmental knee arthroplasty is indicated for the treatment of anteromedial osteoarthritis of the knee with an intact ACL when symptoms are recalcitrant to conservative measures. Mobile bearing unicompartmental knee arthroplasty has demonstrated long-term survivorship of 91% at 20 years, according to a recent report from Price and colleagues.

Exceptionally low wear rates of the polyethylene contribute to the long-term success. In addition, clinical results as measured by Hospital for Special Surgery knee scores are good or excellent in 91% of patients at a minimum follow-up of 10 years, as was documented in 2005 by Price and colleagues.

Coupled with smaller incision surgery and rapid recovery protocols, unicompartmental knee arthroplasty (UKA) is a minimally invasive surgery compared with total knee arthroplasty (TKA). Despite the success of the surgery, there are concerns regarding the increased technical difficulty in performing this operation. New instrumentation has been developed (Oxford Microplasty Instrumentation; Biomet) to facilitate reproducible and consistent execution of the operation, which should ultimately enhance the long-term clinical success.

Setup

A nonsterile tourniquet is placed on the proximal thigh of the operative limb. The operative limb is placed in a hanging leg holder that allows the extremity 30· flexion at the hip and at least 135° of knee flexion. A well-padded foam leg holder supports the contralateral limb. The foot of the bed is dropped perpendicular to the floor.

Three special saws are used to perform the operation: a stiff narrow reciprocating saw, a 12-mm-wide oscillating saw and a double-armed vertical toothbrush saw (Oxford Knee Resection Procedure Three Pack; Synvasive Technologies Inc.). Cement removal is a critical step in the operation and is easier with a Woodson-style curette.

Exposure

The incision is slightly oblique starting from the superomedial edge of the patella to a point 3 cm inferior to the joint line medial to the tibial tubercle. An arthrotomy is performed in line with the skin incision. Proximal extension of 1 cm to 2 cm into the vastus medialis at the level of the superomedial pole of the patella will facilitate the exposure.

The anteromedial aspect of the tibia needs to be exposed to allow for positioning and visualization of the tibial resection guide. It is critical to preserve the medial collateral ligament (MCL). Visualization of the knee is enhanced by removing the anterior horn of the medial meniscus and a small portion of the retropatellar fat pad. If the ACL is absent or non-functional, or if there is full-thickness lateral compartment osteoarthritis, it is recommended to convert to a TKA.

Assuming there is a normal ACL and lateral compartment, the procedure continues by removing osteophytes from the medial and lateral aspects of the intercondylar notch, the medial margin of the medial femoral condyle and the anvil osteophyte anterior to the ACL insertion on the tibia. In addition, removing the anteromedial osteophyte on the tibia serves as a guide for depth of resection for the tibial osteotomy.

Tibial osteotomy

Restoration of the joint space is performed by capturing the medial femoral condyle in the sagittal plane with the preselected size tensioning spoon based on the preoperative templating (Figure 1). Adjustments in tension are made by using sequential 1-mm spoons (range, 1 mm to 3 mm). The proximal tibial coupling clamp is fashioned to the spoon and the proximal tibial resection guide (Figure 2). This clamp comes in 3-mm and 4-mm options, corresponding to the depth of tibial resection and to the expected thickness of the polyethylene at the completion of the procedure.

Figure 1. The sizing spoon is in the medial compartment restoring normal tension in flexion.

Figure 2. The tensioning spoon is linked to tibial resection guide by the coupling clamp.

Figure 3. Resected tibial bone demonstrates anteromedial osteoarthritis with preservation of the posterior cartilage. Images: Morris MJ et al.

Retractor tension is released prior to coupling to prevent inaccurate elevation of the planned resection level. The tibial saw guide is confirmed to be parallel to the long axis of the tibia in the coronal and sagittal planes, which corresponds to 7° of posterior slope utilizing the horizontal tibial resection guide. One pin is used to secure the tibial guide. The MCL is protected with a curved retractor.

The narrow reciprocating saw is used to make the vertical cut at the lateral edge of the MCL and medial edge of the ACL insertion on the tibia with the blade directed at the head of the femur. It is critical to avoid raising the blade during resection, which could violate the posterior tibial cortex below the resection level and place the patient at a higher risk for a tibial plateau fracture.

The horizontal osteotomy is performed with the narrow saw blade, taking care to pass through the posterior cortex. The bone is subsequently removed and should demonstrate anteromedial arthritis with preservation of the posterior cartilage (Figure 3). The remaining meniscus is excised.

Femoral preparation

The femoral canal is opened with a starter drill and awl at a point 1 cm anterior to the anteromedial corner of the intercondylar notch. The intramedullary rod with the distal coupling feature (Figure 4) is placed retrograde up the femoral canal. The metal flexion gap spacer corresponding to the thickness of the flexion space measured by the plastic feeler gauges without tension of retractors on the MCL is positioned in the medial compartment.

Figure 4. This photograph shows the intramedullary rod with cannulated distal coupling feature.

Figure 5. An intramedullary rod is coupled to the flexion gap spacer by the linkage bar.

Figure 6. A 6-mm pilot hole is drilled through the flexion gap spacer.

The coupling bar is used to link the rod to the flexion gap spacer (Figure 5). The 4-mm and 6-mm pilot holes are then drilled sequentially in the distal femoral condyle, with preference to drilling the 10° 6-mm pilot hole, which allows slight flexion of the component to provide more posterior condylar coverage and avoid impingement anteriorly (Figure 6).

The posterior femoral resection guide is placed, and then the posterior condyle is cut with the narrow saw while protecting the MCL with the curved retractor (Figure 7).

Figure 7. This photograph shows the posterior femoral condylar resection guide.

The distal femoral condyle is milled with the zero spigot. Trials are placed, retractors removed and feeler gauges utilized to balance the flexion space. Extension space is balanced in 10° to 20° flexion to relax the posterior capsule. The thickness of the flexion feeler gauge minus the thickness of the extension feeler gauge equals the spigot number that is required to mill the remaining distal femur to balance the flexion/extension gaps.

Trials are then replaced and gaps reassessed. Further milling can be performed, if necessary, to balance the gaps. Anterior femoral condylar bone and posterior osteophytes, potential sources of anterior and posterior impingement in extension and high flexion, respectively, are removed by utilizing the two-in-one resection guide (Figure 8).

Final tibial preparation

It is important to maximize tibial base plate coverage on the plateau. Up to 3 mm of medial overhang is acceptable. The posterior margin of the component is aligned flush with the posterior cortex by placing the universal removal hook into the posterior recess of the knee and pulling it anteriorly to engage the posterior cortex while sliding the tibial component posteriorly to contact the hook.

Figure 8. This photograph shows two-in-one anterior mill and posterior osteophyte resection guide.

It is common and acceptable to have 1 mm to 2 mm of uncoverage anteriorly on the plateau. The component is secured with the tibial template nail. The toothbrush saw is used to remove tibial bone to accept the keel of the prosthesis. The bone is removed with the tibial groove gauge.

The trial components are then placed with the anatomic trial bearing. The bearing should restore the ligaments to normal tension and range of motion should be smooth without impingement or instability.

Implantation of the components

Multiple small holes are made in the femur and tibia to enhance cement interdigitation. Bone surfaces are pulse lavaged and dried. The tibial component is cemented first from posterior to anterior. A Woodson-style curette aids in the removal of excess cement.

Suction is placed on the 4-mm anterior femoral drill hole to provide negative pressure for excellent cement interdigitation while finger impaction of the cement is performed on the femur. The femoral component is placed and excess cement is removed.

The trial polyethylene bearing is inserted, and the leg is placed in 45° of flexion while the cement cures. Confirmation of bearing thickness is performed, and the official bearing snapped into place. A final check of range of motion and tracking should be performed. Routine closure then ensues. Insertion of a drain is left to the discretion of the surgeon.

References:

• Kendrick BJ, Longino D, Pandit H, et al. Polyethylene wear in Oxford unicompartmental knee replacement: a retrieval study of 47 bearings. J Bone Joint Surg (Br). 2009;92:367-373.
• Price AJ, Svard U. A second decade lifetable survival analysis of the Oxford unicompartmental knee arthroplasty. Clin Orthop Relat Res. 2011;469(1):174-179.
• Price AJ, Waite JC, Svard U. Long-term clinical results of the medial Oxford unicompartmental knee arthroplasty. Clin Orthop Relat Res. 2005;435:171-180.
• White SH, Ludkowski PF, Goodfellow JW. Anteromedial osteoarthritis of the knee. J Bone Joint Surg (Br). 1991;73:582-586.

• Michael J. Morris, MD, Keith R. Berend, MD, and Adolph V. Lombardi Jr., MD, FACS, can be reached at Joint Implant Surgeons, Inc., 7277 Smith’s Mill Rd., Ste. 200, New Albany, OH 43054; 614-221-6331; email: MorrisMJ@joint-surgeons.com.
• Disclosures: Morris receives institutional financial research support from Biomet, Inc., and Berend and Lombardi receive institutional financial research support and royalties from and have consulting agreements with Biomet, Inc.