Strains in trussed spine interbody fusion implants are modulated by load and design.

Titanium cages with 3-D printed trussed open-space architectures may provide an opportunity to deliver targeted mechanical behavior in spine interbody fusion devices. The ability to control mechanical strain, at levels known to stimulate an osteogenic response, to the fusion site could lead to development of optimized therapeutic implants that improve clinical outcomes. In this study, cages of varying design (1.00 mm or 0.75 mm diameter struts) were mechanically characterized and compared for multiple compressive load magnitudes in order to determine what impact certain design variables had on localized strain. Each cage was instrumented with small fiducial sphere markers (88 total) at each strut vertex of the truss structure, which comprised of 260 individual struts. Cages were subjected to a 50 N control, 1000 N, or 2000 N compressive load between contoured loading platens in a simulated vertebral fusion condition, during which the cages were imaged using high-resolution micro-CT. The cage was analyzed as a mechanical truss structure, with each strut defined as the connection of two vertex fiducials. The deformation and strain of each strut was determined from 50 N control to 1000 N or 2000 N load by tracking the change in distance between each fiducial marker. As in a truss system, the number of struts in tension (positive strain) and compression (negative strain) were roughly equal, with increased loads resulting in a widened distribution (SD) compared with that at 50 N tare load indicating increased strain magnitudes. Strain distribution increased from 1000 N (+156 ± 415 µÎµ) to 2000 N (+180 ± 605 µÎµ) in 1.00 mm cages, which was similar to 0.75 mm cages (+132 ± 622 µÎµ) at 1000 N load. Strain amplitudes increased 42%, from 346µÎµ at 1000 N to 492µÎµ at 2000 N, for 1.00 mm cages. At 1000 N, strain amplitude in 0.75 mm cages (481µÎµ) was higher by 39% than that in 1.00 mm cages. These amplitudes corresponded to the mechanobiological range of bone homeostasis+formation, with 63 ± 2% (p < .05 vs other groups), 72 ± 3%, and 73 ± 1% of struts within that range for 1.00 mm at 1000 N, 1.00 mm at 2000 N, and 0.75 mm at 1000 N, respectively. The effective compressive modulus for both cage designs was also dependent on strut diameter, with modulus decreasing from 12.1 ± 2.3 GPa (1.25 mm) to 9.2 ± 7.5 GPa (1.00 mm) and 3.8 ± 0.6 GPa (0.75 mm). This study extended past micro-scale mechanical characterization of trussed cages to compare the effects of design on cage mechanical behavior at moderate (1000 N) and strenuous (2000 N) load levels. The findings suggest that future cage designs may be modulated to target desired mechanical strain regimes at physiological loads.

Ex vivo loading of trussed implants for spine fusion induces heterogeneous strains consistent with homeostatic bone mechanobiology.

A truss structure was recently introduced as an interbody fusion cage. As a truss system, some of the connected elements may be in a state of compression and others in tension. This study aimed to quantify both the mean and variance of strut strains in such an implant when loaded in a simulated fusion condition with vertebral body or contoured plastic loading platens ex vivo. Cages were each instrumented with 78 fiducial spheres, loaded between platens (vertebral body or contoured plastic), imaged using high resolution micro-CT, and analyzed for deformation and strain of each of the 221 struts. With repeated loading of a cage by vertebral platens, the distribution (variance, indicated by SD) of strut strains widened from 50N control (4±114µÎµ, mean±SD) to 1000N (-23±273µÎµ) and 2000N (-48±414µÎµ), and between 1000N and 2000N. With similar loading of multiple cages, the strain distribution at 2000N (23±389µÎµ) increased from 50N control. With repeated loading by contoured plastic platens, induced strains at 2000N had a distribution similar to that induced by vertebral platens (84±426µÎµ). In all studies, cages exhibited increases in strut strain amplitude when loaded from 50N to 1000N or 2000N. Correspondingly, at 2000N, 59-64% of struts exhibited strain amplitudes consistent with mechanobiologically-regulated bone homeostasis. At 2000N, vertically-oriented struts exhibited deformation of -2.87±2.04µm and strain of -199±133µÎµ, indicating overall cage compression. Thus, using an ex vivo 3-D experimental biomechanical analysis method, a truss implant can have strains induced by physiological loading that are heterogeneous and of amplitudes consistent with mechanobiological bone homeostasis.

Adolescent Hip Dislocation Combined With Proximal Femoral Physeal Fractures and Epiphysiolysis.

BACKGROUND: The risks and long-term effects of acute hip dislocation combined with proximal femoral physeal fractures and epiphysiolysis have been minimally addressed in the literature. This infrequent combination must be understood to avoid the major complications of complete separation of proximal femoral components during attempted reduction and to predict the probable outcome of surgical treatment. METHODS: Medical records and imaging were retrospectively reviewed to identify patients with a diagnosis of severe to complete slipped capital femoral epiphysis (CFE) or proximal femoral epiphysiolysis in association with hip dislocation. The focus included possible anatomic/vascular disruption and their consequences. RESULTS: Twelve patients were identified. Nine dislocations were posterior; 3 were anterior. In 4 patients, the intact proximal femur was dislocated posteriorly. In 3 patients only the femoral neck was reduced, whereas the CFE remained dislocated. In 1 patient percutaneous pinning was done in the dislocated position before closed reduction. The reduction was successful. In 7 patients only the CFE (4 patients) or femoral neck (3 patients) was displaced at the initial presentation in the emergency room. One patient presented with posterior dislocation associated with complete separation of both components. Ten patients underwent open reduction and internal fixation. Two patients had closed reduction. Nine patients developed complete avascular necrosis, progressive collapse of the femoral head, and degenerative arthritis. Three subsequently had a total hip arthroplasty. One patient developed ischemic change limited to the femoral neck and a nonunion through the epiphysis. One patient had incomplete ischemic necrosis. Only 1 patient had no evidence of ischemic necrosis. CONCLUSIONS: This combination of injuries has several anatomic variations. Leaving the CFE dislocated while reducing only the femoral neck must be avoided. Reduction should be done in the operating room with muscle relaxation. The emergency room is not the venue for reduction. The risk of avascular necrosis is extremely high, whether the separation occurs during the acute dislocation or attempted reduction. LEVEL OF EVIDENCE: Level IV-case series (retrospective review).

Continued growth after limited physeal bridging.

BACKGROUND: After any physeal injury, the primary concern is the possibility of some pattern of growth alteration, particularly transphyseal bridging that may cause lasting deformities and impact subsequent patient care. Small areas of physeal bridging, however, may be associated with continued growth, rather than impairment. METHODS: Seven patients with small central physeal bridges of the distal femur were identified. Demographic data and imaging studies were reviewed. RESULTS: Radiography identified small, relatively centrally located transphyseal osseous bridging that was associated with a linear (longitudinal) region of osseous density extending from the physeal bridge proximally into the metaphysis. This linear striation disappeared at the metaphyseal/diaphyseal gradation, an area of progression proximally from metaphysis to diaphysis. Only 1 patient had a significant leg length inequality. Magnetic resonance imaging confirmed the intrametaphyseal linear sclerotic bone and its disappearance with diaphyseal remodeling. CONCLUSIONS: Small, central transphyseal osseous bridges may form after radiologically confirmed acute physeal injury. Normal physiological (hydrostatic) growth forces can be sufficient to overcome such limited central bridging and allow continued, essentially normal, longitudinal growth. LEVEL OF EVIDENCE: Level IV (retrospective case series); anatomic study.

Scapulothoracic dissociation secondary to major shoulder trauma.

Three skeletally immature patients with scapulothoracic dissociation were reviewed. A 5-year-old child's arm, caught in a conveyor belt, led to complete upper extremity amputation. Multiple fractures, muscular damage, and cutaneous and subcutaneous tissue disruption (degloving) were present throughout the avulsed extremity. Replantation was considered, but not carried out because of these extensive injuries. He was treated with a myoelectric prosthesis. Two older boys had scapulothoracic dissociation (one open, one closed) associated with clavicular diaphyseal fractures following blunt trauma. In each case, the clavicle was stabilized, muscular disruptions were reattached, and bleeding was controlled locally, although no specific major vascular repair was required. There was no return of neurologic function, leaving each patient with a flail upper extremity. One patient and his family eventually elected to have a shoulder disarticulation followed by fitting with a myoelectric prosthesis. The other patient still had a flail extremity at his last evaluation 17 months postinjury, but did not return for subsequent evaluation.

Sever's injury: a stress fracture of the immature calcaneal metaphysis.

Magnetic resonance imaging (MRI) in children with a presumptive diagnosis of Sever's apophysitis and with continuing pain after conservative treatment demonstrated bone bruising within the trabecular bone of the metaphyseal region adjacent to the calcaneal apophysis. Limited portions of the apophyseal secondary ossification center showed similar increased signal changes. MRI studies following treatment with immobilization showed subsidence or disappearance of the metaphyseal but not any apophyseal signal changes commensurate with improvement in symptoms. Accordingly, the disorder commonly referred to as Sever's ''apophysitis'' may be a metaphyseal trabecular stress fracture, similar to the toddler's calcaneal stress fracture that has minimal or no involvement of the apophyseal ossification center, and thus should not be referred to as an apophysitis. Rather, it appears to be an overuse injury causing microinjury within the developing metaphyseal "equivalent" trabecular bone that has not completely adapted to the changing biologic (biomechanical) requirements of the growing, athletically active child.

Nail patella syndrome. A 55-year follow-up of the original description.

The long-term skeletal changes and the lack of significant clinical complaints in a 77-year-old woman with nail patella syndrome are described. Fifty-five years previously she was one of the first reported patients. These early patients came from two families with involvement of multiple individuals with the variable constellation of deformities. We reviewed her skeletal natural history and her family history as it related to nail patella syndrome involvement and treatment, and correlated the original premolecular biology description and subsequent long-term follow-up with the current molecular and genetic concepts of the cause of the variable expression of nail patella syndrome.

A potential role for cell-based therapeutics in the treatment of intervertebral disc herniation.

Lower back pain and disc degeneration negatively affect quality of life and impose an enormous financial burden. An extensive body of scientific work has evolved that characterizes the disc, demonstrating spinal anatomy and morphology that contribute to risk and likely promote failure. Ultimately, matrix failure is responsible for mechanical failure, which in turn results in spinal compromise anatomically and subsequent pain. One intervening approach to breaking this sequence has been to repopulate the anatomy with autologous disc chondrocytes--cells capable of restoring the matrix and retaining the mechanical balance by which the disc functions. This strategy has been implemented both in patients and in animal models, and early results, although preliminary, support the premise as a positive approach.

Supracondylar femur fracture fixation: mechanical comparison of the 95 degrees condylar side plate and screw versus 95 degrees angled blade plate.

The best way to stabilize supracondylar femur fractures remains debatable. Previous studies have compared internal fixation to intramedullary fixation, but none have compared the stiffness characteristics and strength of the 95 degrees angled blade plate (ABP) with the 95 degrees condylar side plate and screw (DCS). 14 synthetic femora were cut in half and the proximal pole of the distal fragment was made secure. A 1 cm gap was made parallel to the femoral condylar weight-bearing surface to create an extraarticular supracondylar femur fracture (OTA 33-A3). 7 femora were stabilized with an ABP and 7 with a DCS. Using an MTS compression/torsion servohydraulic testing machine, each femur was tested in 7 modes of loading: (1) axial compression; (2) anterior compression; (3) posterior compression; (4) medial compression; (5) lateral compression; (6) torsion in external rotation; and (7) torsion in internal rotation. The stiffness of the construct in each mode, the "maximum load in axial compression", and the fatigue characteristics in axial compression were measured. The DCS showed a statistically significant greater stiffness in axial compression and average maximal load than the ABP. The fatigue tests revealed no evidence of permanent deformation or loosening of either construct.