Custom Fixture Research Articles

The Effect of Humeral Component Anteversion on Shoulder Stability with Glenoid Component Retroversion

Posterior glenoid bone loss is often seen in association with glenohumeral osteoarthritis. This posterior asymmetric wear can lead to retroversion of the glenoid component and posterior instability after total shoulder arthroplasty. Options for the treatment of this asymmetric wear include eccentric reaming of the so-called high side, bone-grafting, and/or anteverting the humeral component. Although anteverting the humeral component has been advocated by many, it has not been substantiated on the basis of biomechanical data. The purpose of the present study was to determine whether anteverting the humeral component increases the stability of a total shoulder replacement with a retroverted glenoid component. A total shoulder arthroplasty was performed in eight human cadaveric shoulders. The glenoid component was placed in 15 degrees of retroversion. Two humeral versions were tested for each specimen: anatomic version and 15 degrees of anteversion relative to anatomic version. The specimens were mounted supine in a custom fixture on a servohydraulic testing system. The humerus was translated posteriorly by one-half of the width of the glenoid. Three positions of humeral rotation were tested for each position of humeral version. Both the energy and the peak load were analyzed as measures of joint stability. There was no significant difference in either energy or peak load between the tests performed with the humeral component in 15 degrees of anteversion and those performed with the component in anatomic version in any of the three rotational positions (p > 0.05). Although anteverting the humeral component during total shoulder arthroplasty to compensate for glenoid retroversion has been advocated, these data suggest that compensatory anteversion of the humeral component does not increase the stability of a shoulder replacement with a retroverted glenoid component.

External rotation of the glenohumeral joint: Ligament restraints and muscle effects in the neutral and abducted positions

External rotation of the glenohumeral joint is important in a variety of pathologic states, yet the ligamentous restraints to external rotation have not been thoroughly investigated and the muscle effects have received even less attention. The purpose of this study was to investigate the ligamentous restraints and muscle effects limiting external rotation of the glenohumeral joint in a biomechanical cadaveric model. External rotation torque was applied to the humeri of 15 shoulders mounted in the supine position in a custom fixture while varying rotator cuff and biceps loads in 15 degrees and 60 degrees of glenohumeral abduction. A randomly chosen ligament (coracohumeral ligament [CHL], combined superior and middle glenohumeral ligaments [S+M], anterior band of the inferior glenohumeral ligament [AB], entire inferior glenohumeral ligament [IGHL], or posterior capsule [PC]) was cut, and testing was repeated (n = 3 for each ligament). Torque data were collected every 3 degrees through the entire range of external rotation. Individual muscles were loaded with 22.2 N (designated as a standard state) and were compared with loads of 0, 11.1, and 44.5 N. Alterations in muscle loads were analyzed with a residual maximum likelihood-based repeated-measures model. Ligament effects were analyzed by use of analysis of variance with Tukey correction. In the neutral position, each ligament except the PC significantly affected the torque required for external rotation, with IGHL > CHL > AB > S+M. In this position, loading the subscapularis to 44.5 N significantly increased the torque required to externally rotate the shoulder whereas unloading it significantly decreased the torque required. In the 60 degrees abducted position, each ligament except the PC significantly affected the torque required for external rotation, with IGHL > AB > S+M > CHL. In this position, loading the biceps or subscapularis to 44.5 N significantly increased the torque required to externally rotate the shoulder as rotation increased whereas unloading it significantly decreased the torque required as rotation increased. This work demonstrates that the glenohumeral joint capsule behaves as a cylinder with many regions serving as restraints to external rotation. In addition, the long head of the biceps has an important role as a dynamic restraint to external rotation in the abducted shoulder. This is a new function attributed to the biceps and reinforces the role of external rotation in the generation of biceps and superior labral pathology.

Stress relaxation of bone significantly affects the pull-out behavior of pedicle screws

The initial fixation strength of pedicle screws is commonly tested using a standard pull-out test with load applied at a constant rate. This method overlooks the cyclic nature of in situ loading responsible for clinical failure. This study was undertaken to determine the effects of stress relaxation properties at the bone–screw interface on screw fixation strength. Pedicle screws were inserted into calf lumbar vertebrae using a paired testing array. After embedding and mounting in a custom fixture, axial pull-out tests were performed at the rates of 1, 5, and 25 mm/min. For each vertebra, one screw was pulled at a continuous rate. The other screw was pulled at increments of 0.5 mm, at the same rate, with 1000 s pause between increments. Peak load, energy-to-failure, displacement-to-failure, and stiffness were calculated for each screw pull-out test. Two-way ANOVA showed that the standard pull-out method yielded significantly higher peak loads ( p<0.05) at faster pull-out rates and higher stiffnesses ( p<0.05) at all rates compared to the stress relaxation pull-out protocol. These results suggest that the stress relaxation properties of bone significantly affect the pull-out behavior of pedicle screws, reducing the peak load and stiffness values observed during testing. This mode of testing may provide a better biomechanical model of screw pull-out failure and a more accurate estimate of initial fixation strength.

Use of stereolithography to manufacture critical-sized 3D biodegradable scaffolds for bone ingrowth.

A novel approach to the manufacture of biodegradable polymeric scaffolds for tissue-engineering utilizing stereolithography (SLA) is presented. SLA is a three-dimensional (3D) printing method that uses an ultraviolet laser to photo-crosslink a liquid polymer substrate. The current generation of SLA devices provide a 3D printing resolution of 0.1 mm. The experiments utilized a biodegradable resin mixture of diethyl fumarate (DEF), poly(propylene fumarate) (PPF), and a photoinitiator, bisacylphosphine oxide (BAPO). The PPF is crosslinked with the use of the SLA's UV laser (325-nm wavelength). An SLA device was retrofitted with a custom fixture build tank enclosing an elevator-driven build table. A 3D prototype model testing the manufacturing control this device provides was created in a computer-aided-design package. The resulting geometric data were used to drive the SLA process, and a DEF/PPF prototype part was successfully manufactured. These scaffolds have application in the tissue engineering of bony substrates.

Ligamentous restraints to anterior and posterior translation of the sternoclavicular joint

This experiment was conducted to determine the primary ligamentous restraints to anterior and posterior translation of the sternoclavicular joint. Twenty-four unpaired cadaver specimens were mounted in a custom fixture. Anterior and posterior translations were measured under a sub-failure load in the intact specimen and again after transecting one randomly chosen ligament (anterior capsule, posterior capsule, interclavicular ligament, and costoclavicular ligament; n = 6 for each group). Cutting the posterior capsule resulted in significant increases in anterior translation and posterior translation. Cutting the anterior capsule produced significant increases in anterior translation. Cutting the costoclavicular and interclavicular ligaments had little effect on sternoclavicular joint translation. The posterior capsule is the most important restraint for anterior and posterior translation of the sternoclavicular joint. The anterior capsule is another important restraint for anterior translation. The costoclavicular and interclavicular ligaments have little effect on anterior or posterior translation of the sternoclavicular joint. (J Shoulder Elbow Surg 2002;11:43-7.)