Surgical technique of medial fixed-bearing unicompartmental knee arthroplasty


Intraoperative confirmation of patient candidacy

Even if a unicompartmental knee arthroplasty (UKA) is planned for a patient based on preoperative radiographs and physical examination, the surgeon must confirm at arthrotomy that the patient is an appropriate candidate. Both cruciate ligaments should be intact, although a deficient anterior cruciate ligament (ACL) is occasionally acceptable for fixed-bearing UKA if certain criteria are fulfilled (see Chapter 9 ).

Secondly, there should be no sign of significant mediolateral tibiofemoral subluxation. Evidence of this form of instability is the finding of a so-called “kissing lesion” on the medial aspect of the lateral femoral condyle. This lesion usually consists of a divot secondary to impingement of the lateral tibial spine on the lateral condyle along with an accompanying osteophyte. Although this lesion can usually be seen on a preoperative radiograph, especially a posteroanterior (PA) flexed view, intraoperative assessment is especially important to determine its significance. A large lesion may indicate that medial–lateral stability cannot be restored to the knee by a UKA, whereas a smaller lesion merely requires debridement of the accompanying impinging osteophyte ( Fig. 6.1 ).

Fig. 6.1, Intercondylar femoral osteophytes should be removed to define the anatomy of the medial femoral condyle, relieve any intercondylar impingement, and provide a pathway for a reciprocal saw utilized to complete the tibial resection.

Changes no greater than grade I should be present in the opposite compartment. The patellofemoral compartment can have up to grade III changes, but the presence of eburnated bone in the lateral patellar facet is probably a contraindication to the procedure. Significant inflammatory synovitis is an absolute contraindication especially in the presence of crystalline disease in the form of gout or pseudogout. If there is no synovial inflammation at surgery and no history of inflammatory episodes in the past, many surgeons do not consider crystalline disease a contraindication to UKA.

Generic surgical technique

The technique that follows is as generic as possible regarding fixed-bearing UKA. Each prosthetic design will have individualized features in alignment, cutting jigs, and modes of prosthetic fixation such as lugs or fins. Chapter 7 covers mobile-bearing technique.

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 ).

Fig. 6.2, Preoperative planning for a medial unicompartmental knee arthroplasty involves drawing a conservative resection for total knee arthroplasty with a line 90 degrees to the long axis of the tibia that is 8 to 10 mm below the lateral joint line. The amount of peripheral medial tibial resection is usually 0 to 2 mm, as shown on this radiograph.

Fig. 6.3, A postoperative radiograph showing how a tibial resection of zero from the peripheral medial plateau combined with a 7-mm tibial component corrects the preoperative anatomic alignment.

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 .

Fig. 6.4, Excellent exposure of the medial compartment can be achieved through a short medial arthrotomy with subluxation of the patella using a bent Hohmann retractor.

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