Surgical technique of medial fixed-bearing unicompartmental knee arthroplasty

Basic principles

A significant advantage of UKA is its potentially conservative nature. It preserves both cruciate ligaments, the opposite compartment, and the patellofemoral articulation. If the prosthetic design and surgical technique remain conservative, bone is also preserved in the compartment being resurfaced. The goal should be to prepare a unicompartmental replacement in such a way that no augmentation methods will be necessary at the time of any future revision. The only possible deficiency occurring at revision of a medial UKA should be on the tibial side resulting from subsidence of the tibial component. Fortunately, osteolysis compromising bone stock is extremely rare in UKA.

The following are the basic principles for medial unicompartmental arthroplasty:

• Conservative tibia-first resection

• Assessment of the resultant flexion and extension gaps

• Equalization of the gaps

• Distal femoral resection in the proper alignment and amount

• Sizing the femur and aligning it relative to the tibia in 90 degrees of flexion

• Completion of the femoral preparation

• Sizing, orienting, and completing tibial preparation

• Confirmation of limb alignment and component orientation with trial components

• Implantation of real components

Preoperative planning

To accomplish a conservative tibia-first preparation, the preoperative anteroposterior radiographs should be used to plan the level of the resection. A conservative resection line is drawn on the radiograph at 90 degrees to the long axis of the tibia ( Fig. 6.2 ). The level of this resection is determined on the lateral side 8 to 10 mm below the joint line. The level of the initial tibial resection should be no lower than this line, whether it is for a medial or a lateral compartment arthroplasty. For medial compartment replacement, the resection begins where this line intersects with the most peripheral aspect of the plateau. For most knees, this will be between 0 and 2 mm of peripheral tibial resection. This amount of resection makes sense, because the authors’ experience has shown that for every millimeter of elevation of the joint line from the periphery of the plateau, approximately 1 degree of correction is obtained. Therefore, if the peripheral resection is 0 and a 7-mm tibial component is used, approximately 7 degrees of correction will be achieved. This would convert a typical UKA candidate in 3 degrees of anatomic varus to 4 degrees of anatomic valgus ( Fig. 6.3 ).

Operative exposure

Traditionally, UKA of the medial side was performed by a standard total knee exposure using a median parapatellar arthrotomy with complete eversion of the patella. Care would be taken not to derange the anterior horn of the lateral meniscus. This exposure gave the surgeon the opportunity to completely explore the knee and make an intraoperative decision about whether the patient was a candidate for UKA.

Minimally invasive unicompartmental surgery is now the standard. The shorter incisions permit a shorter hospital stay (possibly even outpatient surgery) and a faster recovery due to less disruption of the quadriceps. If a surgeon prefers a somewhat larger arthrotomy, however, rapid recovery is possible if the patella is subluxed laterally rather than everted.

A shorter than normal skin incision typically used for a TKA is appropriate. It can be approximately 8 to 10 cm in length. The arthrotomy begins just above the superior pole of the patella. It ends distally at the midportion of the tibial tubercle. Adequate inspection of the joint can usually be accomplished by flexing the knee 30 to 40 degrees and manually subluxating the patella. Digital palpation of the patella allows for the detection of eburnated bone on its surface. A retractor such as a bent Hohmann is anchored in the intercondylar notch and allows maintenance of the lateral subluxation of the patella during the procedure ( Fig. 6.4 ). For lateral compartment replacement exposure, see Chapter 8 .