The medial approach is advantageous in that it is the most commonly used and well-known approach in arthroplasty surgery of the knee. It is easy to perform in valgus knee without tight lateral structures and where there is no flexion deformity. Its usefulness in the valgus knee is improved by the technique of intra-articular or “inside-out” lateral structure release.

It is well described by Ranawat et al. [1]. The medial parapatellar arthrotomy is performed followed by minimal subperiosteal liberation of the medial structures just to allow knee exposure and resection of cruciate ligaments and the menisci.

The osteophytes are removed and the tibial cut is made perpendicular to its anatomical axis. The thickness of resected bone of the unaffected medial part should not exceed 6–8 mm.

The femoral distal cut is performed with the use of an intramedullary guide. The femoral valgus angle is decreased from usual 6–3°, which results in a varus cut of the distal femur. This cut is also minimized and should measure no more than 10 mm on the medial condyle, resulting in a thin or almost no lateral condyle cut.

Ligament balancing is performed firstly in extension by decreasing the tightness of the lateral structures as required in a stepwise manner. The order and degree of release vary according to different authors [1, 9–11, 14–16]. The sequence proposed by Ranawat et al. [1] is the one mostly employed.

The tibiofemoral space is opened in extension with the Meary’s spreader. This allows palpation and identification of the tight structures: the posterolateral angle, ITB, and LCL.

Release begins in general with the posterolateral capsule, which is cut at the level of the tibial osteotomy between the posterior border of the ITB and the POP, followed by pie crusting of the capsule above the initial cut (Fig. 10.7). It is at that moment the LCL can be elongated according to Elkus et al. [16]. The POP is preserved unless it is too tight. The ITB is elongated with the pie-crusting technique or is liberated from Gerdy’s tubercle as required.

At least one lateral structure should be preserved in order to maintain lateral stability. If after ligament balancing in extension there is more than 5 mm of mediolateral laxity, a more constrained prosthesis should be used.

The flexion gap is balanced by adapting the thickness of the posterior femoral condyle cut and rotation of the femoral implant. The knee is flexed to 90° and distracted, and the femoral cutting guide is placed in such a way that its posterior edge is parallel to the tibial cut. The observed asymmetry of the posterior condyle cut creates a systemic gap. To achieve appropriate femoral rotation, in addition to using Whiteside’s line as reference, the transepicondylar axis and adequate tensioning in flexion of both compartments should be used [16]. The use of the posterior bicondylar line as reference runs the risk of placement of the femoral implant in internal rotation.

Down- or upsizing of the femoral implant can help in obtaining equality of both spaces. When mediolateral stability in different degrees of flexion is achieved, and after verifying the absence of notching of the anterior femoral cortex, the cutting guide can be fixed and final cuts performed.

The advantage of the medial approach is the choice of which structures are released and in which sequence. The disadvantages are related to weakening of the medial structures and limited access to posterolateral side due to the lateral subluxation of the extensor apparatus, which increases external rotation of the tibia. However, the sliding lateral condyle osteotomy (SLCO) is possible through this approach [17]. Access to the medial structures is easy if medial condyle osteotomy or tensioning of MCL is required.

The lateral approach is confusing and more difficult because of anatomical landmark inversion and different management of the soft tissues and the patella. It is, however, logical, giving direct access to the tight lateral structures and conserving the medial structures. It does not allow for tensioning procedures of the MCL [18], which are nonetheless exceptional. On the other hand, it facilitates lateral procedures such as the SLCO [12].

This approach, popularized by Keblish et al. [19] and Buechel et al. [7], follows the lateral border of patellar tendon in its distal part. Keblish proposed a lateral parapatellar arthrotomy with plasty of the lateral femoropatellar retinaculum and the use of the retropatellar fat pad during closure. Mertl et al. [20] pointed out the utility of tubercle osteotomy.

The skin incision is midline, centered on the patella, or slightly eccentric, following its lateral border. The transquadricipital arthrotomy starts proximally between the vastus lateralis and rectus femoris muscles and runs distally to the superolateral corner of the patella. Sectioning of the lateral retinaculum is done in a “Z-plasty” fashion, by separating the retinacular and capsular layers. The retinaculum is sectioned about 3 cm laterally from the patella’s edge, whereas the capsule is cut just next to it [19]. The retropatellar fat pad is detached from the patellar tendon and attached to the lateral capsular flap. This allows expansion of the lateral capsular layer and facilitates closure [7, 19]. The capsule is incised distally up to Gerdy’s tubercle. The ITB is detached, maintaining continuity with the anterior sural aponeurosis, and soft tissue subperiosteal detachment is continued to the proximal tibiofibular articulation. Special attention must be paid to the common peroneal nerve. In this manner the ITB is loosened, and lateral structure (LCL, POP, and posterolateral capsule) release may be performed depending on the degree of tightness of the involved structures (Fig. 10.8). Initial simultaneous release of both the LCL and POP should be avoided as it results in significant laxity. It may, however, be indispensable in cases with significant flexion deformity.

In general, the patella is inverted, rarely dislocated. In the situation of patella baja or the danger of distal peel off of extensor apparatus, a tibial tubercle osteotomy or detensioning of the quadricipital tendon by the “rectus snip” technique should be considered. A tibial tubercle osteotomy is preferred to patella tendon detachment, which has an uncertain prognosis.