Ligament releases are necessary for contemporary non-conforming femoral-tibial articulations.

Most total knee arthroplasty prostheses are designed to be non-conforming at the articulation between the femoral and tibial components. This design is chosen on the arthroplasty principle that “constraint causes loosening” and conforming surfaces have been considered constrained. To provide stability the ligaments are adjusted so that tension in the ligament can provide stability for the total knee replacement.

Ligament releases are NOT necessary for contemporary conforming femoral-tibial articulations.

Through the majority of the range of motion, the normal human knee is not stabilised by ligament tension. Rather, it is the geometrical conformity of the femur and tibia, especially on the medial side, that provides stability. The ligaments are present and ready to restrain the knee from excess varus-valgus or anterior-posterior loads. In a knee design that is congruent, ligaments may be left intact as in the normal knee, ready to provide restraint but not necessarily to provide stability except when excess loads are applied to the knee.

When designing and using the ADVANCE Medial Pivot total knee, the author has left ligaments in the toe-region of the stress-strain curve rather than releasing and tensioning the ligaments. Patient satisfaction survey data at routine follow-up visits for patients at 7–15 years after arthroplasty with this type of reconstruction indicate high satisfaction despite medial and lateral opening (on valgus and varus stress) that would be considered “mid-flexion instability” for non-conforming joints that require careful ligament releases and tensioning.

Background. Patellofemoral complications have dwindled with contemporary total knee designs that market anatomic trochlear grooves that intend to preserve normal patella kinematics. While most reports of patellofemoral complications address patella and its replacement approach, they do not focus on shape of trochlear grooves in different prostheses [1]. The purpose of this study was to characterize 3D geometry of trochlear grooves of contemporary total knee designs (NexGen, Genesis II, Logic, and Attune) defined in terms of sulcus angle and medial-lateral offset with respect to midline of femoral component in coronal view and to compare to those of native femurs derived from 20 osteoarthritic patient CT scans. Materials and Methods. Using 3D models of each implant and native femur, sulcus location and orientation were obtained by fitting a spline to connect sulcus points marked at 90°, 105°, 130°, and 145° of femoral flexion (Fig A). Implant reference plane orientations were established using inner facets of distal and posterior flanges. Reference planes of native femurs were defined using protocols developed by Eckhoff et al. [2] where coronal plane was defined using femoral posterior condyles and greater trochanter. In the coronal plane, a best fit line was used to measure sulcus angle and medial-lateral offset with respect to midline at the base of trochlear groove (Fig B). Results. With exception to Logic (0° sulcus angle & 0 mm offset), contemporary knee designs include high valgus angulations (4° to 18°) with laterally-biased offsets (3 to 5 mm). The native sulcus angle on average was slightly valgus, but varied significantly among the cohort (−0.2° ± 4.6°). Native trochlear groove offset was biased laterally (2.5 ± 1.7 mm). Discussion. We observed a considerable geometric deviation between native femur and implants in terms of sulcus angle while both geometries displayed comparable lateral bias at the base of trochlear groove. Similar to past studies by Iranpour et al. [4] and Feinstein et al. [5], a large variation in sulcus angle was observed among the selected native femurs with an average of small valgus angulation (Fig C). However, most contemporary trochlear grooves are biased towards higher valgus angulations. Retrieval and registry studies have shown that NexGen trochlear groove design (4° sulcus angle) has been shown to better accommodate natural patellas, which highlights that the differences among designs may be significant [5,6]. It remains unclear which features specifically translate to better patellafemoral outcomes, which is a merit for further study. Conflict of Interest: None. Figure A. Sulcus points defined at various flexion angles. Figure B. Best fit line to measure ML-offset and sulcus angle. Figure C. Sulcus angle comparison to past studies. For any figures or tables, please contact authors directly (see Info & Metrics tab above).

Aims

Medial pivot (MP) total knee arthroplasties (TKAs) were designed to mimic native knee kinematics with their deep medial congruent fitting of the tibia to the femur almost like a ball-on-socket, and a flat lateral part. GMK Sphere is a novel MP implant. Our primary aim was to study the migration pattern of the tibial tray of this TKA.

Restoring the overall mechanical alignment to neutral has been the gold standard since the 1970s and remains the current standard of knee arthroplasty today. Recently, there has been renewed interest in alternative alignment goals that place implants in a more “physiologic” position with the hope of improving clinical outcomes. Anywhere from 10 – 20% of patients are dissatisfied after knee replacement surgery and while the cause is multifactorial, some believe that it is related to changing native alignment and an oblique joint line (the concept of constitutional varus) to a single target of mechanical neutral alignment. In addition, recent studies have challenged the long held belief that total knee placed outside the classic “safe zone” of +/− 3 degrees increases the risk of mechanical failure which theoretically supports investigating alternative, more patient specific, alignment targets. From a biomechanical, implant retrieval, and clinical outcomes perspective, mechanical alignment should remain the gold standard for TKA. Varus tibias regardless of overall alignment pattern show increased polyethylene wear and varus loading increases the risk of posteromedial collapse. While recently questioned, the evidence states that alignment does matter. When you combine contemporary knee designs placed in varus with an overweight population (which is the majority of TKA patients) the failure rate increases exponentially when compared to neutral alignment. A recent meta-analysis on mechanical alignment and survivorship clearly demonstrated reduced survivorship for varus-aligned total knees. The only way to justify the biomechanical risks associated with placing components in an alternative alignment target is a significant clinical outcome benefit but the evidence is lacking. A randomised control trial comparing mechanical alignment (MA) and kinematic alignment (KA) found a significant improvement in clinical outcomes and knee function in KA patients at 2 year follow-up. In contrast, Young et al. recently published a randomised control trial comparing PSI KA and computer assisted mechanical TKA and found no difference in any clinical outcome measure. Why were the clinical outcomes scores in the MA patients so different: One potential explanation is that different surgical techniques were used. In the Dosset study, the femur was cut at 5 degrees valgus in all patients and femoral component rotation was always set at 3 degrees externally rotated to the posterior condylar axis. We know from several studies that this method leads to inaccuracies in both coronal plane and axial plane in some patients. Young et al. used computer assisted navigation to align his distal femur cut with the mechanical axis and adjusted femoral component rotation to the transepicondylar axis. The results suggest that a well performed mechanical aligned total knee replacement has excellent clinical performance equal to that of kinematic alignment without any of the long term risks of implant failure. Most contemporary TKA implants are designed to be loaded perpendicular to the polyethylene surface and placing them in shear without extensive biomechanical testing to support this alignment target may put patients at long term risk for an unproven benefit. Have we not learned our lesson?

Aims

Wear of the polyethylene (PE) tibial insert of total knee arthroplasty (TKA) increases the risk of revision surgery with a significant cost burden on the healthcare system. This study quantifies wear performance of tibial inserts in a large and diverse series of retrieved TKAs to evaluate the effect of factors related to the patient, knee design, and bearing material on tibial insert wear performance.