A biomechanical assessment of fixation methods for a coronoid prosthesis.

BACKGROUND: The coronoid process is an integral component for maintaining elbow joint stability. When fixation of a fracture is not possible, prosthetic replacement may be a feasible solution for restoring stability. The purpose of this in-vitro biomechanical study was to compare fixation methods for a coronoid implant. METHODS: A coronoid prosthesis was subjected to distally-directed tip loading after implantation using four fixation methods: press-fit, anterior-to-posterior screws, posterior-to-anterior screws, and cement. Testing was performed on seven fresh-frozen ulnae in a repeated-measures model. Rounds of cyclic loading were applied at 1 Hz, for 100 cycles, increased in 50 N increments up to a maximum of 400 N. Micro-motion of the implant was quantified using an optical-tracking system. Outcome variables included total displacement, distal translation, gapping, anterior translation and axial stem rotation. FINDINGS: Cement fixation reduced implant micro-motion compared to screw fixation, while the greatest implant micro-motion was observed in press-fit fixation. Comparing screw-fixation techniques, posterior-anterior screws provided superior stability only in distal translation. The implant did not experience displacements exceeding 0.9 mm with screw or cement fixation. INTERPRETATION: Cement fixation provides the best initial fixation for a coronoid implant. However, the stability provided by both methods of screw fixation may be sufficient to allow osseous integration to be achieved for long-term fixation. Large displacements were observed using the press-fit fixation technique, suggesting that modifications would need to be developed and tested before this technique could be recommended for clinical application.

Reconstruction of the coronoid process using the tip of the ipsilateral olecranon.

BACKGROUND: Autograft reconstruction of the coronoid using the tip of the olecranon has been described as a treatment option for comminuted coronoid fractures or coronoid nonunions that are not repairable. The purpose of this in vitro biomechanical study of the coronoid-deficient elbow was to determine whether coronoid reconstruction using the tip of the ipsilateral olecranon would restore elbow kinematics. METHODS: An elbow motion simulator was used to perform active and passive extension of six cadaveric arms in the horizontal, valgus, varus, and vertical orientations. Elbow kinematics were quantified with use of the screw displacement axis of the ulna with respect to the humerus. Testing was performed with an intact coronoid, a 40% coronoid deficiency, and a coronoid reconstruction using the tip of the ipsilateral olecranon. RESULTS: Creation of a 40% coronoid deficiency resulted in significant changes (range, 3.6° to 10.9°) in the angular deviations of the screw displacement axis relative to the intact state during simulated active and passive extension in the varus orientation with the forearm in pronation and in supination (p < 0.05). Reconstruction of the coronoid using the ipsilateral olecranon tip restored the angular deviations to those in the intact state (p > 0.05) with the arm in all orientations except valgus, in which there was a small but significant difference (0.4° ± 0.2°, p = 0.04) during passive motion with forearm supination. CONCLUSIONS: Reconstruction of the coronoid using the tip of the ipsilateral olecranon was an effective method for restoring normal kinematics over a range of elbow motion from 20° to 120° in a cadaveric model of an elbow with a 40% coronoid deficiency. This reconstruction technique may prove beneficial for patients with elbow instability due to coronoid deficiency.

The effect of a coronoid prosthesis on restoring stability to the coronoid-deficient elbow: a biomechanical study.

PURPOSE: The coronoid process has been recognized as a critical component in maintaining elbow stability. In the case of comminuted coronoid fractures, where repair is not possible or has failed, a prosthesis may be beneficial in restoring the osseous integrity of the elbow joint. The hypothesis of this in vitro biomechanical study was that a coronoid prosthesis would restore stability to the coronoid-deficient elbow. METHODS: An anatomically shaped metallic coronoid prosthesis was designed and developed based on computed tomography-derived measurements and optimized to account for average cartilage thickness. Elbow kinematics and stability were determined for 8 cadaveric arms in active and passive elbow flexion in the varus, valgus, horizontal, and vertical positions using an elbow motion simulator. Varus-valgus angulation and internal-external rotation of the ulna relative to the humerus were quantified in the intact state, after collateral ligament sectioning and repair (control state), after a simulated 40% transverse coronoid fracture, and after implantation of the coronoid prosthesis. RESULTS: Internal rotation of the ulna increased with a 40% coronoid fracture in the horizontal and varus positions. Increases in varus angulation after coronoid fracture were also observed in the horizontal and varus positions, during active and passive flexion, respectively. Following implantation of the coronoid prosthesis, elbow kinematics were restored similar to control levels in all elbow positions. CONCLUSIONS: Our findings support our hypothesis that an anatomically shaped coronoid prosthesis would be effective in restoring stability to the coronoid-deficient elbow. CLINICAL RELEVANCE: This study provides evidence that the use of an anatomical implant restores stability to the coronoid-deficient elbow and rationale for further study and development of this method. For comminuted coronoid fractures, where repair is not possible or has failed, our research indicates that a prosthesis may be a feasible treatment option.

Selecting the diameter of a radial head implant: an assessment of local landmarks.

INTRODUCTION: Little information exists on radial head implant diameter sizing methods. When the native head is absent due to extensive comminution or previous excision, the lesser sigmoid notch may be a useful landmark for sizing. We evaluated the reliability of native radial head measurements, and the lesser sigmoid notch, as landmarks for radial head implant diameter sizing. METHODS: We examined 27 fresh frozen ulnae and their corresponding radial heads. The maximum, minimum, and dish diameters of the radial heads were measured. A radial head implant diameter was selected based on the congruency of the trial implants with the radius of curvature of the lesser sigmoid notch. Intraobserver and interobserver reliability for all measurements and implant selection were assessed using intraclass correlation coefficients (ICC). Correlations between the native radial head measurements and the selected radial head implant diameter or the lesser sigmoid notch radius of curvature were assessed using the Pearson correlation coefficient (PCC). RESULTS: Radial head diameter measurements demonstrated strong to excellent intraobserver (ICC ≥ 0.75) and interobserver reliability (ICC ≥ 0.82). The lesser sigmoid notch sizing method showed poor interobserver reliability (ICC = 0.34). Only a moderate correlation was found between the native radial head and the lesser sigmoid notch (PCC ≤ 0.80) or the selected radial head implant size (PCC ≤ 0.59). CONCLUSION: Radial head diameter measurements showed excellent reliability, suggesting that the excised radial head, when available, should be used to select the implant diameter. The reliability of using the lesser sigmoid notch for sizing the diameter of radial head implants was only moderate, suggesting this is an unreliable landmark for implant diameter sizing.

Rehabilitation of the medial- and lateral collateral ligament-deficient elbow: an in vitro biomechanical study.

DESIGN: In vitro biomechanical research using an elbow motion simulator. INTRODUCTION: The optimal rehabilitation of elbow dislocations with medial collateral ligament (MCL) and lateral collateral ligament (LCL) injuries has not been defined. PURPOSE: To determine a safe rehabilitation protocol for elbow dislocations with MCL and LCL injuries. METHODS: Eight cadaveric elbows underwent simulated active and passive motions with the arm in multiple orientations. Varus-valgus angulation and internal-external rotation of the ulna relative to the humerus were quantified for the intact joint and with injured MCL and LCL. RESULTS: Active motion with injured MCL and LCL in the horizontal and vertical orientations resulted in kinematics similar to the intact elbow, whereas passive motion resulted in significant kinematic alterations. Marked elbow instability was noted in the varus and valgus orientations using both active and passive motion. CONCLUSIONS: Elbows with MCL and LCL injuries should be rehabilitated using active motion in the horizontal or vertical orientations. LEVEL OF EVIDENCE: Basic science research.

Reconstruction of the coronoid using an extended prosthesis: an in vitro biomechanical study.

BACKGROUND: When repair of comminuted coronoid fractures is not possible, prosthetic replacement may restore elbow stability. The purpose of this biomechanical study was to determine whether a coronoid implant with an extended tip would improve elbow stability compared with an anatomic prosthesis in the setting of collateral ligament insufficiency. MATERIALS AND METHODS: Passive elbow extension was performed in 7 cadaveric arms in the varus and valgus positions and active and passive extension in the horizontal position by use of an elbow motion simulator. Varus-valgus laxity of the ulna relative to the humerus was quantified with a tracking system with a native coronoid, a 40% coronoid deficiency, an anatomic prosthesis, and an extended prosthesis, with the collateral ligaments sectioned and repaired. RESULTS: Laxity significantly increased after a 40% coronoid deficiency with both repaired and sectioned collateral ligaments (P ≤ .01). With the ligaments repaired, there was no significant difference in laxity between the native coronoid, the anatomic implant, or the extended implant. Ligament sectioning alone produced severe instability, with a mean laxity of 42.75° ± 11.54° (P < .01). With insufficient ligaments, the anatomic prosthesis produced no change in laxity compared with the native coronoid, whereas the extended implant significantly reduced laxity by 21.56° ± 17.70° (P = .02). CONCLUSIONS: An anatomic coronoid implant with ligament repair restores stability to the coronoid-deficient elbow to intact levels. In the setting of ligament insufficiency, an extended implant improves stability relative to an anatomic implant, but the elbow remains significantly less stable than an intact elbow. Studies are needed to evaluate the feasibility of these designs.