Robotic-assisted total knee arthroplasty improves accuracy and precision compared to conventional techniques

Aims Robotic-assisted total knee arthroplasty (RA-TKA) is theoretically more accurate for component positioning than TKA performed with mechanical instruments (M-TKA). Furthermore, the ability to incorporate soft-tissue laxity data into the plan prior to bone resection should reduce variability between the planned polyethylene thickness and the final implanted polyethylene. The purpose of this study was to compare accuracy to plan for component positioning and precision, as demonstrated by deviation from plan for polyethylene insert thickness in measured-resection RA-TKA versus M-TKA. Methods A total of 220 consecutive primary TKAs between May 2016 and November 2018, performed by a single surgeon, were reviewed. Planned coronal plane component alignment and overall limb alignment were all 0° to the mechanical axis; tibial posterior slope was 2°; and polyethylene thickness was 9 mm. For RA-TKA, individual component position was adjusted to assist gap-balancing but planned coronal plane alignment for the femoral and tibial components and overall limb alignment remained 0 ± 3°; planned tibial posterior slope was 1.5°. Mean deviations from plan for each parameter were compared between groups for positioning and size and outliers were assessed. Results In all, 103 M-TKAs and 96 RA-TKAs were included. In RA-TKA versus M-TKA, respectively: mean femoral positioning (0.9° (SD 1.2°) vs 1.7° (SD 1.1°)), mean tibial positioning (0.3° (SD 0.9°) vs 1.3° (SD 1.0°)), mean posterior tibial slope (-0.3° (SD 1.3°) vs 1.7° (SD 1.1°)), and mean mechanical axis limb alignment (1.0° (SD 1.7°) vs 2.7° (SD 1.9°)) all deviated significantly less from the plan (all p < 0.001); significantly fewer knees required a distal femoral recut (10 (10%) vs 22 (22%), p = 0.033); and deviation from planned polyethylene thickness was significantly less (1.4 mm (SD 1.6) vs 2.7 mm (SD 2.2), p < 0.001). Conclusion RA-TKA is significantly more accurate and precise in planning both component positioning and final polyethylene insert thickness. Future studies should investigate whether this increased accuracy and precision has an impact on clinical outcomes. The greater accuracy and reproducibility of RA-TKA may be important as precise new goals for component positioning are developed and can be further individualized to the patient. Cite this article: Bone Joint J 2021;103-B(6 Supple A):74–80.

Factors Affecting Stability of the Reverse Total Shoulder Arthroplasty: A Cadaveric Biomechanical Study

Background: The overall objective of this study was to investigate whether a reverse shoulder arthroplasty could provide adequate stability to a shoulder even with extreme soft tissue loss. The specific objectives of this study were: to determine if just the deltoid, conjoined tendon, and triceps are sufficient soft tissues to allow a Reverse Shoulder Arthroplasty (RSA) to provide shoulder stability and to determine the influence of load direction, rotation, shoulder position, and polyethylene thickness on RSA stability in this soft-tissue deficient model. Methods: This study utilized six cadaveric shoulders that had all soft tissue removed, with the exception of the deltoid, conjoint tendon, and long head of triceps. A reverse shoulder arthroplasty was then performed (Delta III, DePuy Inc., Warsaw, IN) and an increasing dislocation force was applied perpendicular to the humeral socket centerline until dislocation occurred, or a maximum load of 100 N was reached. This was repeated to measure the effect of four factors: load direction, arm position, polyethylene thickness, and arm rotation on force to dislocation. Results: For load direction, there was an increase in force to dislocate an inferior load direction (p=0.01). There was a lower not dislocated percentage and lower survival for a posterior load direction (p=0.02). For arm position, there was a decrease in force for dislocation and lower survival for both abduction and extension arm positions. There was a higher not dislocated percentage for a flexion arm position (p=0.01). For arm rotation, there was a lower not dislocated percentage and lower survival for an external rotation arm position (p=0.03). There was no statistically significant influence of polyethylene thickness (p=0.26). Conclusion: The deltoid, conjoined tendon, and triceps are sufficient to stabilize an RSA. Load direction, arm position, and arm rotation were all shown to significantly affect stability. Finally, polyethylene thickness may not affect overall RSA stability in this soft-tissue deficient model. Level of Evidence: Basic science study, Biomechanical study.

Combining sensor and robotic technologies to achieve a well balanced total knee arthroplasty while avoiding any soft tissue releases

Achieving a balanced total knee throughout the entire range of motion leads to improved patient reported outcomes and satisfaction. Sensor-assisted technology allows the surgeon to quantitatively assess and address imbalance through either soft tissue releases or bone recuts. However, balancing through soft tissue releases leads to unpredictable gap increments and frequent early over-releases.METHODS: During a consecutive and prospective series of 29 robotic total knee surgeries, intra-operative load sensors were used following the initial bone resections to quantitatively assess the knee’s state of balance through the range of motion with trial components in place. Load measurements were taken at 10 and 90 degrees of knee flexion. Based on previous literature, a balanced knee is defined as having a mediolateral load difference below 15 pounds (lbf) through the range of motion, with an absolute load magnitude per compartment above 5lbf and not exceeding 45lbf. The initial load numbers were recorded as well as the number and type of subsequent corrections needed to achieve quantitative balance.RESULTS Of the 29 robotics cases, only 12 (41%) were well-balanced after the initial bone cuts (mechanical alignment by measured resection). Another two cases were too loose and required an increase in the polyethylene thickness size of two millimeters to achieve a well-balanced knee without further bone resection. In 14 cases, a bone recut was required to balance the knee. More specifically, four cases required a recut of the femur, ten cases required a recut of the tibia. Eventually, one case was left unbalanced in flexion with a mediolateral load differential of 20 lbf. It should be noted explicitly that no soft tissue releases were done for any of the 29 cases. At the end, all 29 knees were considered well balanced in extension and all but six (79%) at 90° of flexion. For these six cases with balance issue at 90° of flexion, absolute load magnitude in both compartments was below 45 lbf and above 5lbf, though the mediolateral load differential was between 15lbf and 30lbf.DISCUSSION Based on a preliminary series, this work demonstrates the opportunity of combining multiple technologies to achieve a quantitatively balanced knee through the range of motion without any soft tissue release.

Influence of Polyethylene Thickness on Axis Pin in Linked Elbow Implant

Elbow joint failure results in weakness, instability, pain and loss of motion. Total Elbow replacement is last way to relief pain and instability. One of available complications after total elbow replacement is bushing wear. Excessive bushing wear may cause instability, pain and inflammatory reply to implant loosening. New prosthetic designs is probably responsible for positive changes to decrease complications. The aim of the present study was to analyze effect of ulnar bushing thickness on transmitted stress to axis pin in linked elbow implant. Axis pin, humeral/ulnar components and polyethylene bushings were modeled. All materials were assumed linear, homogenous, elastic and isotropic. A 50 N force was applied in tip of ulnar component. Distal and proximal of humeral component were fixed. Results showed maximum stress at 94 MPa for axis pin contacting ulnar bushing with 10mm outer diameter while values were 111MPa and 138MPa for ulnar bushing with 9mm and 8mm outer diameters respectively. The present study demonstrates that changing bushing thickness affects stress of axis pin.