Biomechanical comparison of C1-C2 posterior arthrodesis techniques.

STUDY DESIGN: Biomechanical comparison of 5 atlantoaxial posterior arthrodesis techniques. OBJECTIVE: To assess the relative value of different posterior wire constructs when one or two transarticular screws are used. SUMMARY OF BACKGROUND DATA: A combination of Gallie or Brooks techniques and 2 posterior transarticular screws has been shown to be effective for atlantoaxial arthrodesis. Anatomic constraints may preclude the insertion of a transarticular screw unilaterally or bilaterally. METHODS: Ten adult human cadaveric upper cervical spine specimens were used. The specimens were tested intact, after odontoidectomy and transverse and capsular ligament section and after stabilization with each of the 5 techniques: Brooks-Jenkins cable fixation, Brooks-Jenkins with unilateral transarticular screw, Gallie posterior wire construct with unilateral transarticular screw, Brooks-Jenkins with bilateral screws, and Gallie with bilateral screws. Pure moments were applied in flexion-extension, lateral bending, and torsion within physiologic limits (<1.5 Nm). RESULTS: In flexion-extension and lateral bending, the range of motion (ROM) and neutral zone (NZ) increased significantly after the specimens were injured as compared with intact spines (P < 0.001). After stabilization, the ROM and NZ were significantly lower than in injured and intact spines in all motions (P < 0.01) except lateral bending in the intact spine. Among the 5 instrumented techniques, the ROM for the Gallie construct with 1 screw was significantly higher than for the Brooks-Jenkins construct with 1 or 2 screws in flexion-extension (P < 0.05). In axial torsion, the Gallie construct with 1 screw displayed a larger NZ and ROM than any of the other 4 constructs (P < 0.05). CONCLUSIONS: Gallie or Brooks-Jenkins cable fixation alone may not be adequate for atlantoaxial arthrodesis. If 2 supplemented transarticular screws can beinserted, there is no difference between the Gallie or Brooks techniques. If only a single screw can be inserted, the Brooks-Jenkins technique is recommended rather than a Gallie technique.

Medial collateral ligament strain with partial posteromedial olecranon resection. A biomechanical study.

BACKGROUND: Partial resection of the posteromedial aspect of the olecranon in the treatment of valgus extension impingement osteophytosis is a well-described technique. It has been hypothesized that removal of the normal olecranon process, beyond the osteophytic margin, increases the strain in the anterior bundle of the medial collateral ligament. METHODS: We used an electromagnetic tracking device to investigate the strain in the anterior bundle of the medial collateral ligament as a function of increasing applied torque and posteromedial resections of the olecranon in seven cadaveric elbows. Applied torques under valgus stress consisted of hand weight, hand weight plus 1.75 Nm, and hand weight plus 3.5 Nm. Resections were conducted in sequential 3-mm increments, from 0 to 9 mm. We measured changes in the length of the anterior and posterior bands of the anterior bundle of the medial collateral ligament with strain gauges. The strains of the two bands were averaged, and the average was reported. RESULTS: The strain in the anterior bundle of the medial collateral ligament was found to increase with increasing flexion angle, valgus torque, and olecranon resection beyond 3 mm. In two elbows, the anterior bundle of the medial collateral ligament ruptured during testing following the 9-mm resection. There was a significant difference between the strain following the 6-mm resection and that following the 3-mm resection at 110 degrees of flexion with 3.5 Nm of added torque (p = 0.004). CONCLUSIONS: In this in vitro cadaver study, an increase in flexion angle, an increase in valgus torque, and resection of > or =6 mm led to an increase in strain in the anterior bundle of the medial collateral ligament. The non-uniform change in strain related to 3 mm of resection suggests that resections of the posteromedial aspect of the olecranon of >3 mm may jeopardize the function of the anterior bundle.

The noncircular shape of the radial head.

PURPOSE: The purpose of this study was to define the shape of the radial head by identifying the relationship between precisely defined axes of the radial head. METHODS: An anatomic study was done to define the shape of the radial head and specifically the relationship between the long and the short axis. Twenty-seven cadaveric upper extremities were used. The x and y axes of the radial head were defined in relationship to the radial notch of the ulna, with the forearm in neutral position. Outer diameters of the x and y axis were measured. These were compared with the actual maximum and minimum diameters of the radial head. X and y diameters of the articulating surface of the radial head also were measured. RESULTS: Paired 1-tailed Student's t-tests were used to compare the x and y diameters of the radial head. Regression analysis of x and y diameters of the radial head was done to identify a correlation between these parameters.Paired 1-tailed Student's t-tests showed a significant difference between X and Y diameters of the radial head. Regression analysis of x and y diameters of the radial head showed a strong correlation between these 2 axes. CONCLUSIONS: The radial head is not round. A strong correlation exists between the x and y diameters of the radial head. The orientation of the long axis is perpendicular to the radial notch with the forearm in neutral rotation. This finding will make it possible to approach the anatomy of the radial head more closely when designing radial head prostheses. The definition of the axes can be used as a guide when implanting the radial head prosthesis.

Kinematics and functional characteristics of the Pritchard ERS unlinked total elbow arthroplasty.

This study examined the kinematic characteristics of the Pritchard ERS elbow-resurfacing system, with special attention paid to the effects of the radial head component. The kinematics between the ulna and humerus were assessed in 6 human cadaveric specimens by an electromagnetic tracking system throughout a full flexion/extension range of motion. The elbows were studied under 2 loading conditions, in 3 orientations (neutral, varus, and valgus), and under 4 surgical conditions. The varus/valgus and internal/external rotation laxities were used to assess the condition differences. Specifically, the maximum laxities throughout the extension motion were compared, as were the laxities at 40 degrees, 75 degrees, and 110 degrees of flexion. Both the varus/valgus and internal/external rotation laxities of the ulnohumeral joint increased after total elbow arthroplasty (TEA) implantation, with and without a radial head. This increase was most evident in the extension portion of the arc of motion. At 40 degrees of flexion, the varus/valgus laxity of the intact elbow was 4 degrees +/- 2 degrees versus 11 degrees +/- 8 degrees for a TEA with a radial head and 22 degrees +/- 11 degrees for a TEA without a radial head while the elbow was being subjected to compressive loads via the biceps, brachialis, and triceps. The kinematic data demonstrate a consistent increase in laxity with the Pritchard ERS TEA. They also indicate that a radial head component is necessary for optimal tracking and stability of the ERS arthroplasty.

Kinematic and torque-related effects of dorsally angulated distal radius fractures and the distal radial ulnar joint.

PURPOSE: The purpose of this study was to examine the torque required to achieve a full range of motion of the distal radioulnar joint (DRUJ) as a result of increasing dorsal angulation from simulated fractures of the distal radius. Based on this study the accepted amount of dorsal angulation of the distal radius can be determined. METHODS: In 9 fresh cadaver limbs motion of the DRUJ was simulated by a custom motion and loading forearm device. The malunion model of the distal radius was controlled by a specially designed external fixation frame that provided control in 6 degrees of dorsal angulations (N, 0 degrees, 10 degrees, 20 degrees, 30 degrees, and 40 degrees ). The study included an intact and nonintact triangular fibrocartilage complex. RESULTS: This study showed that torque across the DRUJ was affected by the degree of simulated malunion of the distal radius. With more than 30 degrees dorsal angulation the torque across the DRUJ was increased in both muscle loading and unloading conditions. Although significance was not noted, with resistive loading this study showed torque changes with as little as 10 degrees malunion of the distal radius. CONCLUSIONS: We conclude that reduction of distal radius fractures to within 10 degrees of dorsal angulation is needed to allow patients to maintain full forearm and wrist rotation.

Biomechanical evaluation of posterior screw fixation in cadaveric cervical spines.

Sixteen fresh-frozen spines from cadavers (C4-T1) were randomized on the basis of dual energy xray absorptiometry analysis of bone mineral density. The specimens were subjected to physiologic loads (

Biomechanical evaluation of new posterior occipitocervical instrumentation system.

Twelve fresh-frozen cadaveric occipitocervical specimens were randomized based on dual energy xray absorptiometry analysis of bone mineral density. The specimens were subjected to physiologic loads in a device that applied pure unconstrained flexion and extension, lateral bending, and axial rotational moments. The spines were tested intact and after major injury simulating transoral decompression of the dens. Biomechanical testing using pure moments with physiologic loads (< 1.5 N-m) was used to compare stability of posterior occipitocervical plates and screws, loop and cable construct, and new cervical rod and screw system. The injury created significantly less stiffness and greater range of motion and neutral zone at C1-C2 in flexion and extension and lateral bending and greater range of motion and neutral zone in axial rotation than the intact state. In lateral bending, the new rod construct had significantly lower mean values for range of motion than the loop and the plate construct. In axial rotation, the rod construct had a significantly higher mean value for stiffness than the other two devices and a significantly lower mean value for range of motion than the loop. The new rod-based instrumentation system for occipitocervical fixation is biomechanically equivalent or superior to a plate and screw construct and a rod and cable system.

Dynamic stabilizing function of the deltoid muscle in shoulders with anterior instability.

BACKGROUND: The stabilizing role of the deltoid muscle has not been extensively studied. PURPOSE: To study the contribution of the deltoid muscle to anterior stability of the shoulder. STUDY DESIGN: Controlled laboratory study. METHODS: We used nine fresh cadaveric shoulders with the arm at 90 degrees of abduction and 90 degrees of external rotation. The position of the humeral head was monitored by an electromagnetic tracking device with 0 and 1.5 kg of anterior translation force; with 0, 1, 3, and 5 kg of force applied to each of the anterior, middle, and posterior portions of the deltoid muscle; and with the capsule intact, vented, and with a simulated Bankart lesion. RESULTS: With the capsule intact, anterior displacement was significantly reduced by application of load to the middle deltoid muscle. After the capsule was vented, load application to the anterior, middle, or posterior deltoid muscle significantly reduced anterior displacement. With a simulated Bankart lesion, effects of muscle loading were most apparent: anterior displacement was significantly reduced with loading of each muscle portion. CONCLUSION: The deltoid muscle is an anterior stabilizer of the glenohumeral joint with the arm in abduction and external rotation. CLINICAL RELEVANCE: The stabilizing function of the deltoid muscle takes on more importance as the shoulder becomes unstable.

Importance of a radial head component in Sorbie unlinked total elbow arthroplasty.

The effects of a radial head component on total elbow arthroplasty kinematics and stability were evaluated using an anatomic design unlinked total elbow prosthesis. An electromagnetic tracking device recorded motion and varus and valgus displacements under various conditions in 10 cadaveric elbows. The motion patterns of the intact elbows and the Sorbie-Questor total elbow prostheses with a radial head component were similar, as both tended to have a valgus position in extension, varus at midflexion, and more valgus toward full flexion. Under conditions of simulated muscle loading, the maximum valgus and varus laxity of the elbow prosthesis was, on average, 8.6 degrees +/- 4.0 degrees greater than normal. Without the radial head component, however, significant kinematic disturbances and instabilities were seen. The varus and valgus displacements were 13.3 degrees +/- 5.5 degrees greater than the intact elbows. One total elbow arthroplasty without a radial head dislocated during testing. Increasing the muscle loading across the elbow significantly enhanced dynamic stability of the total elbow arthroplasties, especially in the extension half of elbow motion where instability is greatest. However, this dynamic enhancement of stability was seen only in those elbows in which the radial head component had been implanted. The radial head component is an important stabilizer, particularly in extension for this prosthesis, and possibly for other unlinked total elbow prostheses. Although instability of unlinked prostheses depends on the prosthetic design, the use of a radial head replacement may be an important factor in preventing such instability. Perhaps even more importantly, a radial head component balances the load distribution across the articulation, which could decrease stress on the ulnohumeral articulation and therefore possibly reduce polyethylene wear, osteolysis, and loosening.

Analysis of dynamic distal radioulnar convergence after ulnar head resection and endoprosthesis implantation.

The most common method to treat the arthralgic distal radioulnar joint is resection of the entire ulnar head (Darrach procedure). Pain and weak grip strength usually manifest complications related to instability of the distal forearm. In an attempt to mechanically stabilize the distal forearm after ulnar head resection, an endoprosthesis was developed to replace the ulnar head after Darrach resection. The goals of this study were to evaluate the dynamic effects of the Darrach procedure on radioulnar convergence and the mechanical efficacy of implantation of an ulnar head endoprosthesis after Darrach resection. Cadaver forearms were rotated actively and passively and relevant muscles were loaded simultaneously with a dynamic PC-controlled forearm simulator. Resultant total forearm torque and 3-dimensional kinematics of the ulna, radius, and third metacarpal were recorded simultaneously in 7 fresh-frozen cadaver upper extremities. Comparisons were made between the intact state, after Darrach resection, and after implantation of ulnar head endoprosthesis. The Darrach resection created substantial forearm instability with movement of the radius ulnarly (0.92-0.38 cm vs intact state) and anteroposterior translation in each loading condition. Implantation of the ulnar head endoprosthesis effectively restored distal radioulnar joint stability by simulating the geometry of the ulnar head, further stabilized by attaching the triangular fibrocartilage complex. These laboratory data provide validity to implanting an ulnar head endoprosthesis to stabilize the distal forearm after Darrach resection.

An analysis of the constraint properties of the distal radioulnar ligament attachments to the ulna.

Nine cadaver upper extremities were tested to evaluate the constraint properties of the dorsal and palmar radioulnar ligaments at their foveal and styloid attachments to the ulna. The specimens were tested by anterior and posterior displacement of the radius relative to the ulna with the forearm in pronation, supination, and neutral rotation. There were no statistically significant differences in the relative percent of constraint contribution of the styloid and foveal ligament insertions. In neutral forearm rotation total displacement after sectioning both ligament insertions tended to be larger in palmar than in dorsal displacement. A similar trend was observed with 60 degrees forearm supination. In the pronated position, however, the trend was reversed with larger total displacement in dorsal displacement.