Bertrand W. Parcells, MD, and Alfred J. Tria Jr., MD
Authors’ Disclosure Statement: Dr. Tria reports that he receives royalties from Smith & Nephew, and is a consultant for Smith & Nephew, Medtronic, and Pacira. Dr. Parcells reports no actual or potential conflict of interest in relation to this article.
The BCS TKA utilizes a modified cam-post articulation to provide both anterior and posterior stabilization (Figure 4).46 The surgical approach remains the same and the implant geometry affects the motion. The BCS TKA design demonstrates femoral rollback at 90° with an average of 14 mm for the MFC and 23 mm for the LFC, and 10° internal tibial rotation.46,47 Additionally, it provides increased sagittal stability during early flexion and an improved pivot shift (indicating improved anterior stabilization).
The BCR designs preserve both cruciates and provide anterior and posterior stabilization. Fluoroscopic imaging has demonstrated contact points in full extension, and posterior rollback at 90° flexion that more closely replicates the normal knee.48
Design and Surgical Techniques for Bicruciate Knee Replacements
If all of the ligaments are preserved, the TKA surfaces must allow motion to be driven by the ligaments in combination with the surfaces alone. The femur can be designed anatomically with asymmetric condyles. The femoral box must allow for preservation of the tibial bone island without impinging upon the cruciate ligaments. The tibial surface must be minimally constrained with concavity medially and convexity laterally.
The bone island preservation does not permit a single-piece tibial polyethylene insert. Therefore, the inserts will replicate the UKA designs (Figure 5). The knee should allow greater range of motion with the possibility of heel to buttocks contact. This increased motion will lead to greater roll back of the femur on the tibia and can lead to subluxation of the femoral runner off of the tibial surface on the lateral side, mimicking the normal knee. This subluxation is desirable but may lead to increased wear of the polyethylene on the lateral side of the knee.
The instruments should be specific for the design but must also be user-friendly. The 2 major issues with the surgery are balancing the knee in full extension and flexion, and preservation of the tibial bone island. The preexisting knee deformity should be <10° in all planes to limit the amount of collateral ligament releases. The collaterals must be balanced in a similar fashion to the standard TKA. Flexion contracture can be treated with posterior capsular release around the cruciates or with an increased distal femoral resection (2 mm at the maximum).
It is important to size the femur correctly because it will be difficult to adjust the flexion gap on the tibial side. A 9-mm posterior medial femoral condyle resection is a reasonable guide if the condyle is not atrophic. However, the exact resection thickness will be implant-specific and should be correlated with the dimensions of the prosthesis being implanted. The tibial bone island must be properly rotated with respect to the center line (Akagi’s line)49 and must not be undercut. The tibial instrument should include pins or blocks to prevent the sawblades from undercutting the island (Figure 6), as undermining leads to fracture in full extension. If undermining occurs, it may be possible to place a cancellous screw through the island and still preserve the ligaments. The integrity of the island is best tested by bringing the knee to full extension and checking for liftoff of the bone. If there is significant compromise of the island, the bone should be resected and either a CR or PS TKA can be implanted. Della Valle and colleagues50 reported a 9.2% incidence (11 of 119 cases) of bone island fracture in their early experience with a BCR TKA and improved this to 1.9% (5/258 cases) after reassessing their technique.
The gap tension should be evaluated either with traditional spacer blocks or with tensioning devices on the medial and lateral side of the knee after the tibial resections are completed. The polyethylene inserts are anatomically different. It may be possible to vary the thickness from medial to lateral, but not in excess of 2 mm.
As the BCR surgical techniques evolve, the balancing and tibial resection may be refined through specialized instrumentation. Such “smart instruments” that incorporate gyros may expedite tibial alignment, and sensor devices may assist with gap balancing. Haptic surgical robotic guides may assist in the tibial resection, facilitating bone island preservation by avoiding any possibility of undermining. At present these assistive aides are not necessary for the operation but may play a future role.
Clinical Results of Knee Arthroplasties
The results of knee replacements improved steadily from the 1970s through the 1990s. The scoring systems were somewhat limited and there was little data on the perception of the patients. The prosthetic designs stabilized at the end of the 1990s with only minor modifications since the year 2000. The 20-year results show similar findings for both the CR and the PS designs. There is little evidence to suggest a clinical correlation with the observed kinematic differences between CR and PS TKA designs.40,51-58 Multiple studies have demonstrated equivalent range of motion38,39,59 and subjective outcome measures (Table 1).60 A randomized prospective trial that compared kinematics and functional scores between the 2 designs failed to observe significant differences in function despite differences in kinematics.46 Equivalence in clinical outcome was further supported by a Cochrane Review meta-analysis that evaluated 1810 patients in 17 selected studies.61 The Knee Society scores have all been in the 92% to 95% ratings with survivals between 90% and 95%.
