While originally utilized for elderly, low-demand with good outcomes, the use of total knee arthroplasty (TKA) for younger and more active patients is becoming more common. In addition to patients with higher demands, longer life expectancy, and higher expectations, there also remains a significant percentage of patients with a level of dissatisfaction following TKA.¹,²,³ While many theories exist as to the reason for dissatisfaction following TKA, no one single reason has been identified. Ultimately patients with less significant arthritis, younger age, higher activity levels, higher demands, or higher expectations tend to fall into this dissatisfied category.⁴,⁵ Surgeons, implant manufacturers, and other thoughtful professionals working toward improving outcomes have looked at implant design as a potential source of either dissatisfaction, or opportunity for improvement.

Historical discussions regarding implant designs that retain or eliminate the cruciate ligaments have discussed the PCL primarily. Cruciate-retaining (CR) and posterior-substituting (PS) TKA designs have been the mainstay in knee arthroplasty design for the past many decades, with data that continue to fail to demonstrate superiority of one design over the other.⁶ Recent interest has developed regarding the potential for retention of both cruciates, including the anterior cruciate ligament (ACL), to potentially improve outcomes and address the issues of dissatisfaction.⁷,⁸,⁹,¹⁰

The ACL has rarely been discussed as a structure of importance in TKA. Recent studies have demonstrated that many patients undergoing routine TKA maintain an intact anterior cruciate ligament.¹¹ If retaining the anterior cruciate ligament during TKA had the potential to re-create more normal knee kinematics and create a knee that feels more natural, it is possible that the patient could be more satisfied with their TKA. Retention of native soft tissues for knee stability, as opposed to traditional reliance on stability from the implant, could additionally afford decreased mechanical wear of the implant. Decreased prosthetic constraint with more normal kinematics and perception in a bicruciate TKA could potentially help address issues of dissatisfaction and even implant survivorship, with appropriate implant technologies and materials.

With all of these theoretical benefits, it is important to understand the background and current understanding of bicruciate TKA.

While the concept of bicruciate-retaining (BCR) knee arthroplasty at first seems to be a novel concept, knee arthroplasty designs as early as the 1960s included implants that preserved both cruciate ligaments. Gunston Polycentric Knee was among the first to develop a BCR knee arthroplasty, with the goal of more naturally mimicking native knee kinematics.¹² The Polycentric Knee utilized independent cemented semicircular stainless-steel caps of the femoral condyles and independent polyethylene components on the medial and lateral tibia. Later designs, such as the Duocondylar and the Geomedic Knee, evolved toward a single femoral component to improve femoral preparation and continued to preserve the ACL,¹³,¹⁴,¹⁵ though tibial preparation and implants, along with durable tibial fixation, remained problematic. While early survivorship looked promising in these earliest BCR TKA designs, possibly related to the decreased level of constraint, long-term results, like many of the early TKA designs, were not as good, with limitations in materials properties, implant design, and surgical techniques leading to loosening, polyethylene wear, osteolysis, patellofemoral problems, and poor postoperative motion.¹⁶,¹⁷

Over time, along with increasing and improving material properties of implants, additional bicruciate TKAs were designed and released to the market. Among the most successful of those included Cloutier bicruciate TKA (Hermes 2C).¹⁸,¹⁹ While successful clinically, the implant was not widely accepted due to the complex ligament balancing and surgical technique.

The work on BCR TKA described by Pritchett using Townley Anatomic TKA and later the TKO bicruciate knee (BioPro, Port Huron, MI, USA) probably lead to the most recent enthusiasm in bicruciate TKAs.²⁰ With long-term survivorship reported at 23 years of 89% for all-cause revision, and 94% when eliminating polyethylene wear, which was the most common cause of failure in the early design, their results appear promising. However, his further publications that showed significant patient preferences of bicruciate knees compared to CR or PS knees
offered perhaps the most compelling reasons to revisit bicruciate TKA as a potential opportunity to improve patient satisfaction following TKA.²⁰,²¹

In order to understand the potential benefit of saving the ACL during TKA, it may be helpful to understand its function and the kinematics in the native and replaced knee. The native knee is a complex joint with multiple static and dynamic structures all working together to guide motion and provide stability. The cruciate ligaments provide both rotational as well as translational stability throughout the arc of motion. The ACL, with its two bundles, help to limit anterior tibial translation and guide knee rotation. The anteromedial bundle becomes tighter in flexion while the posterolateral bundle is taut in extension.²² The screw home mechanism further aids stability in extension as the tibia externally rotates relative to the femur due to asymmetry between the femoral condyles and restraint from the taut ACL.²³ With flexion, femoral rollback occurs as the PCL tightens and drives the femoral condyles posteriorly on the tibia and internally rotates the tibia. In addition to improving flexion ability, the rollback further enhances the lever arm of the quadriceps muscle and patellar tracking.²⁴

Beyond the guided motion afforded by the cruciates, the ACL also likely provides proprioceptive sensory feedback in the knee as it has been shown to contain mechanoreceptors.²⁵ In the setting of arthroplasty, work by Fuchs et al²⁶ compared proprioception between a BCR arthroplasty to healthy controls and concluded that a “total knee arthroplasty that retains all intraarticular ligaments achieves proprioceptive results comparable with healthy subjects.” Advocates of BCR TKA have suggested that improved outcomes may be realized both due to the potential for more normal kinematics as well as a more natural feel of the artificial knee that results from the retained proprioception, which may be more similar to that seen in unicompartmental arthroplasty.²⁷

In order to perform a BCR TKA, the presence of the ACL would seem to be an obvious requisite of the surgery. Common perception of knees undergoing TKA surgery is that an ACL is often absent. Contrary to this belief, however, Johnson et al²⁸ found the presence of an intact ACL in patients undergoing TKA in 78% of patients. Lachman exam under anesthesia had a poor sensitivity when used alone at 33%. MRI had 90% sensitivity when a reading of indeterminate was considered to be intact. Sagittal wear was also evaluated on a lateral radiograph, and anterior wear of the medial tibial plateau had an intact ACL and all patients with posterior medial tibial wear had an incompetent ACL. Prior to that, Sabouret et al¹⁹ had concluded in their series that the presence of a functional ACL, even if degenerated or frayed, was adequate for the carrying out of BCR TKA.