James R. Brownhill

Improved accuracy of computer assisted glenoid implantation in total shoulder arthroplasty: an in-vitro randomized controlled trial.

BACKGROUND Glenoid replacement is challenging due to the difficult joint exposure and visualization of anatomical reference landmarks. Improper positioning of the glenoid component or inadequate correction of the retroversion using currently available instrumentation may lead to early failure. The objective of this study was to evaluate a computer-assisted technique to achieve a more accurate placement of the glenoid component compared to traditional techniques. METHODS Sixteen paired cadaveric shoulders were randomized to either traditional or computer-assisted glenoid implantation. Preoperative planning consisting of CT scanning with 3-dimensional image modeling of the shoulder specimens and intraoperative tracking with real-time feedback provided to the surgeon was employed in the computer-assisted group. A validated, previously published, standardized protocol for tracking the orientation of the glenoid in space using 3 glenoid surface landmarks was employed. All phases of glenoid implantation (initial guide pin insertion, reaming, drilling of the peg holes, and final component implantation) were tracked and recorded by the computer. A post-implantation CT scan was performed in both groups to compare how accurately the implants were placed. RESULTS The computer-assisted technique was more accurate in achieving the correct version during all phases of glenoid implantation and as measured on the post-implantation CT scan (P < .05). The largest errors with traditional glenoid implantation were observed during drilling and, more so, during reaming. The trend was to overly retrovert the glenoid. CONCLUSIONS Computer assisted navigation results in a more accurate glenoid component placement relative to traditional techniques. LEVEL OF EVIDENCE Basic Science Study.

Morphologic analysis of the proximal ulna with special interest in elbow implant sizing and alignment

A better understanding of the morphology of the proximal ulna should permit the development of ulnar component designs which have an improved fit to the native bone, thus leading to more accurate implant positioning. Computed-tomography (CT) scans of 31 cadaveric proximal ulnae were analyzed using computer aided design software to determine the shape of the medullary canal relative to the articular surface. The diameter, curvature, cross-sectional centroid position, and coronal and sagittal angulation of the ulnar canal were all calculated with respect to the center of the greater sigmoid notch. Posterior and lateral offsets increased distally from the articulation center, and the mean diameter of the canal was larger in males than in females (P < .05). The average valgus angulation was 8.0 +/- 4.0 degrees for males and 7.2 +/- 3.1 degrees for females (P = .6). Longer stemmed ulnar implants may require a modular design to meet anatomic constraints during implant positioning.

The effect of implant malalignment on joint loading in total elbow arthroplasty: an in vitro study

HYPOTHESIS Aseptic loosening is one of the leading causes of failure in total elbow arthroplasty. Incorrect implant positioning and alignment in other joints such as the knee have been found to lead to excessive loading and wear. Although similar alignment difficulties exist in the elbow, the effect of implant malalignment on wear-inducing loads is not yet known. This in vitro study determined the effect of anterior malpositioning and varus-valgus and internal-external malrotations on humeral stem loading in total elbow arthroplasty. METHODS AND MATERIALS Computer-navigated linked elbow arthroplasty was conducted in 8 cadaveric elbows. A modular, instrumented humeral component was used to measure loading during simulated elbow motion while the position of the ulna relative to the humerus was recorded. RESULTS Loading increased for all malaligned implant positions tested (P < .05). During simulation of implant malpositioning, combinations of internal-external and varus-valgus malrotations that tended to preserve the line of action of the elbow flexors had lower loads than combinations that did not. DISCUSSION This in vitro study showed that loading does increase after humeral component malalignment; however, further studies are required to determine the long-term effects on polyethylene wear and component loosening.

A comparison of registration techniques for computer- and image-assisted elbow surgery.

Optimal function following elbow replacement surgery is dependent on the accurate replication of the elbows flexion-extension axis. Currently, position and orientation of the axis are estimated from visual landmarks. In order to develop computer-assisted techniques to more accurately define this axis, a surface-based registration technique employing a hand-held laser scanner was evaluated against a conventional paired-point registration method to determine whether it produced improved alignment of the flexion-extension axis of the elbow. Registration error was 0.8 ± 0.3 mm for surface-based registration, compared with 1.9 ± 1.0 mm for the conventional registration method. These results suggest that the implementation of a surface-based registration technique may lead to a more accurate axis determination and improved clinical outcomes following elbow replacement surgery.

Defining the flexion-extension axis of the ulna: implications for intra-operative elbow alignment.

The increased utilization of total elbow replacements has resulted in a correspondingly increased number of failed implants requiring revision. The most common reason for revision is aseptic loosening of the ulnar component due to polyethylene induced osteolysis. Implant malalignment is thought to be an important cause of bearing wear and implant failure. The ulnar flexion axis can be used to accurately align the ulnar component of the elbow implant; however, the optimal method of determining this axis intra-operatively is unknown. This in vitro study determined the relationship amongst kinematically and anatomically defined ulnar flexion axes in an effort to improve the accuracy of ulnar component positioning. Five different techniques were used to determine the ulnar flexion axis in 12 cadaveric specimens, 3 kinematic and 2 anatomic. The techniques were compared with the screw displacement axis from simulated elbow flexion. An anatomic measurement technique using the guiding ridge of the greater sigmoid notch of the ulna and the radial head was found to most accurately replicate the position and orientation of the screw displacement axis of the elbow (p<0.05). Because an anatomically derived flexion axis can be determined using both pre-operative imaging techniques, as well as with intra-operative guides, it is more practical than kinematically derived techniques requiring tracking systems for clinical application and should provide reliable and consistent results.

Kinematics and laxity of a linked total elbow arthroplasty following computer navigated implant positioning

Aseptic loosening in total elbow arthroplasty (TEA) remains the most common cause of long-term failure. While several different mechanisms of implant loosening have been suggested, it is likely that one important underlying cause is implant malpositioning, resulting in changes in joint kinematics and loading. Although use of computer navigation has been shown to improve component positioning in other joints, no such system currently exists for the elbow. This study used real-time computer feedback for humeral, ulnar, and radial component positioning in 11 cadaveric extremities. An elbow motion simulator evaluated joint kinematics. Endosteal abutment of the stems of the humeral and ulnar components precluded optimal positioning in 5 and 6 specimens, respectively. Loss of the normal valgus angulation following elbow arthroplasty (p < 0.05) suggests that errors in humeral component positioning translate directly into changes in joint kinematics during active motion. These findings suggest that although computer navigation can reproduce normal joint kinematics, optimal implant positioning may require a TEA system which allows for some modularity to accommodate the normal variations in osseous morphology of the elbow.

The effect of implant linking and ligament integrity on humeral loading of a convertible total elbow arthroplasty

Background Both unlinked and linked total elbow arthroplasty (TEA) implants have been employed with no consensus as to the optimal design. The present study aimed to evaluate the effect of collateral ligament integrity and implant linkage on wear-inducing loads in a convertible TEA. Methods Eight fresh frozen upper extremities were tested in an elbow motion simulator. A convertible TEA with an instrumented humeral stem was inserted using computer navigation. Elbow kinematics and humeral loading were recorded with the TEA both linked and unlinked. The collateral ligaments were then sectioned and testing was repeated. Results In the dependent position, there was no effect of implant linkage or ligament sectioning on humeral loading. Humeral loading was significantly greater following sectioning of the collateral ligaments but not after linking the TEA with the arm in the valgus position. Humeral loading was significantly greater after linking the TEA but not after sectioning of the collateral ligaments and with the arm in the varus position. Conclusions Collateral ligament integrity reduces wear-inducing loads for both an unlinked and linked TEA. Linkage of a convertible TEA increases humeral loading, which may have detrimental effects on implant longevity.