Current methods of atlantoaxial stabilization rely on screw fixation. Screw placement may be transarticular, or C1 lateral mass screws in combination with pedicle or pars screws at C2. These techniques can put the vertebral artery at risk. There is also dissection around the ganglion of the second cervical nerve root, which can lead to significant bleeding or postoperative pain.

The natural history of Anterior Cruciate rupture is one of progressive deterioration of knee function, with the development of instability, meniscal tears and post traumatic osteoarthritis. The current surgical approach is for anatomical reconstruction using a biological tissue autograft. It is well understood that the initial stability is dependent on the strength of the fixation rather than the strength of the graft, until the graft becomes biologically incorporated in the bone tunnel. A study was carried out to better understand postoperative internal bone stresses in anterior cruciate ligament (ACL) reconstruction surgery. The mechanical aspects of an interface screw fixation were examined both experimentally and numerically, with the aim to minimize deleterious effects in ACL reconstruction. The tibial cortical/cancellous bony tunnel and the stress pattern resulting from the screw fixation in the tunnel are investigated.

The human lumbar intervertebral disc helps stabilize the vertebral bodies by absorbing and distributing loads acting on the spine while maintaining motion across the joint [1]. Current testing methodologies have been designed to study rigid mechanical devices (plates, rods, screws, mechanical joints), and are not suitable for assessing the overall ability of a compliant structure, like the intervertebral disc, to interact with the surrounding tissue structures and restore functionality to the joint [2]. The objective of this study was to develop a kinematics-based testing protocol that prescribes a known kinematics input to a lumbar spinal motion segment unit (MSU) and measures the capacity of the intact MSU to accommodate the motion in terms of compressive and shear loads and range of motion.

Long posterior fusion constructs in the lumbar spine cause substantial posteriorly directed loading of the supporting pedicle screws, particularly during patient bending activities. Although there are numerous documented accounts of clinical failure at the pedicle screw-bone interface [1,2], the in situ pull-out strength of pedicle screws in long surgical constructs has not been characterized. Previous biomechanical studies have quantified pedicle screw pull-out force in cadaveric models through destructive testing or in nondestructive cases, through the use of custom-machined pedicle screws instrumented with strain gages [3–6]. However, these techniques involve altering screw geometry and may fail to properly simulate in vivo mechanical loading conditions. The goal of this study was to develop and validate a sensor system for measuring pedicle screw pull-out forces in long posterior constructs in situ during multi-segmental cadaveric testing.

While proximal humerus fractures are relatively infrequent and the majority are amenable to non-operative treatment, open reduction with internal fixation remains a surgical mainstay [1,2]. With the advent of locking fixation, the majority of proximal humerus fractures requiring surgery are treated with locking plates. Biomechanical studies have demonstrated that locked plating is superior to intramedullary fixation, conventional or blade plating in two and three part proximal humerus fractures [3,4]. Despite the increased stability and use of locking fixation, loss of reduction remains a post-operative problem in osteoporotic bone with comminuted fractures [5]. Presently we hypothesized that: 1) Medial comminution of the calcar region is a determinant of fracture and fixation stability; and 2) Greater stability and increased resistance to varus deformity is gained through use of locking screws that cross the inferior surgical neck and penetrate the humeral head, i.e., the calcar region of the humerus.

A femoral intramedullary nail is a metal rod which is inserted into the femoral canal to stabilize fractures of the long bone. The retrograde femoral nail is a type of intramedullary nail which is inserted at the distal end of the long bone and is typically used for distal femur fractures. While the success rates are high, some complications still exist, including failure of the nail through the screw holes, failure or loosening of screws, and bone fracture [1]. There are many design variables that affect clinical performance including the implant material, nail and screw diameters, the number of screws, and the nail and screw position and orientation. To help determine the most suitable design for a given fracture the effect of the above variables on the stresses and displacements in the nail, screws, and bone may be important to consider.

The recent trend in internal fixation of fractures puts a greater emphasis on biological fixation, which involves the use of locking plates and screws [1]. While conventional plates rely on frictional forces between the plate and the bone, locking plates utilize a threaded connection between screw and plate for fixation and stability [2]. Surgeons choose either type of fixation device in treatments for specific fractures and injuries.

Cervical fusion is a traditional surgical technique in the management of spondylotic pathologies. An increased rate of arthrodesis has been well stated in the literature by using anterior and/or posterior instrumentation. Despite excellent results for the multi-level cervical fusions, failures due to the pseudarthrosis, graft dislodgement, migration and screw loosening were reported. These failures were also found to be directly proportional to the number of fused levels. The multi-level fusions with a single strut graft (corpectomy) have only two graft-endplate interfaces and a lower rate of pseudarthrosis failures. But it has a longer lever arm and moment, thus disrupting the normal sagittal alignment of the cervical spine. On the other hand, the multi-level fusion with multiple inter-body grafts (discectomy) maintains the sagittal alignment, but a higher rate of pseudarthrosis failures were expected due to a large number of graft-endplate interfaces. Some investigators have advocated a combination of corpectomy and discectomy, while others believe to perform either one of them due to the individual advantages and disadvantages as per their patient needs. Consequently, a dilemma and controversy still exists in the selection of the type of reconstructive fusion technique. The objective of the present study was to compare the biomechanical stability of the three reconstructive fusion techniques — corpectomy, discectomy and combined corpectomy-discectomy. The stability of the superior motion segment was compared to the inferior motion segment to determine the direction of propagation of the adjacent segment disease.

Background: The interference screw is widely used to fix the graft in the tibial portion in anterior cruciate ligament (ACL) reconstruction. However, there is no evidence of what type of screws, if metallic or bioabsorbable, have the better mechanical performance. Methods: Twenty porcine tibias, 10 for metallic and 10 for bioabsorbable screw, were submitted to cyclic loading between 50 N to 250 N for 1000 cycles at 1 Hz. For devices that not failed during cyclic loading, a tensile test to failure was performed at strain rate of 50 mm/min. Results: There were no differences in the stiffness and ultimate failure load between metallic and bioabsorbable screw. Conclusion: There is no difference in biomechanical performance between metallic and bioabsorbable screw.

In order to widen a patient’s upper jaw, or maxilla, orthodontists will make use of a maxillary expansion (ME) appliance. This procedure is typically used in adolescents having a narrow maxillary complex. Widening the jaw may be achieved through a variety of means such as expansion screws or springs [1]; however, they are all utilized to generate additional space allowing for correction of various tooth misalignments. An example of a common appliance is provided in Fig. 1a.

Osteoporosis is a critical challenge in orthopedic surgery. Osteoporotic patients have an increased risk of loosening and failure of implant constructs due to a weaker bone-implant interface than with healthy bone. Pullout strength of pedicle screws is enhanced by increased screw insertion depth. However, more knowledge is needed to define optimal pedicle screw insertion depth in relation to screw-bone interface biomechanics and the resulting loosening risk. This study evaluates the effects of screw length on loosening risk in the osteoporotic lumbar spine.

Optimization of pedicle screw insertion depth for ideal fixation and fusion remains a clinical challenge. Improved screw purchase may improve fixation strength 1 , which is especially critical in an osteoporotic patient population. Extended screw insertion depths, up to and through the anterior cortex, have yet to be compared to more commonly used shorter pedicle screws in a laboratory controlled series of experiments. The purpose of this study is to evaluate screw purchase in the osteoporotic lumbar spine as a function of insertion depth, which may be used to optimize pedicle screw-rod constructs.

Lower lumbar lordosis plays a critical role in sagittal alignment. It has been shown that restoration of lumbar lordosis in patients with preoperative sagittal imbalance is necessary to prevent postoperative sagittal decompensation [1]. Further, restoration of lower lumbar lordosis in patients with degenerative flatback syndrome has been shown clinically to result in additional correction of the thoracic curve and sacral slope [2]. Currently, there are three commonly used intraoperative techniques to restore lumbar lordosis: (1) cantilever bending, (2) in situ bending, and (3) compression and/or distraction of screws along posterior fusion rods. Although powerful, all three techniques require the surgeon to impart large forces to the accompanying posterior fusion hardware, often causing failure of hardware and inconsistency to achieve pre-operatively planned lordosis. To date, there has been no clinical or biomechanical study to address the comparative performance of these three techniques. In efforts to determine a standard of care for sagittal alignment via lumbar lordosis restoration, the goal of this study is to establish a relation between the three techniques, and the resulting demands on posterior fusion hardware. It is hypothesized that greater loads will be observed in the hardware during in situ bending, increasing the risk of pedicle screw pullout.

The ankle joint, comprised of the distal ends of the tibia and fibula as well as talus, is key in permitting movement of the foot and restricting excessive motion during weight-bearing activities. Medial ankle injury occurs as a result of pronation-abduction or pronation-external rotation loading scenarios in which avulsion of the medial malleolus or rupture of the deltoid ligament can result if the force is sufficient [1]. If left untreated, the joint may experience more severe conditions like osteoarthritis [2]. To avoid such consequences, medial ankle injuries — specifically bony injuries — are treated with open reduction and internal fixation via the use of plates, screws, wires, or some combination thereof [1, 3–4]. In this investigation, the mechanical performance of two such devices was compared by creating a 3-dimensional model of an earlier cadaveric study [5], validating the model against the cadaveric data via finite element analysis (FEA), and comparing regions of high stress to regions of experimental failure.

The Achilles tendon is the largest tendon in the human body. It connects the gastrosoleus complex to the calcaneus bone in the foot. It thus allows for transfer of force and heel elevation during forward propulsion of the foot in gait. It is also one the most commonly injured tendons. (1). Acute repair or acute intervention for non-operatively treated tendons do well to restore function. However, chronic Achilles tendon ruptures often require tendon transfer rather than attempt at primary repair. The treatment considered the gold standard is the transfer of the flexor hallucis longus (FHL) tendon to the Achilles insertion of the calcaneus (2).

A small but significant fraction of screw placement in the proximal femur for hip fractures results in iatrogenic fractures associated with the screw hole [1]. There is evidence that the failure occurs under fatigue, and it is likely that those individuals experiencing failure have greater stresses in the proximal femur as dictated by key morphometric variables, such as cortical thickness and bone size. Finite element analysis (FEA) could greatly facilitate fracture risk assessment in the presurgical phase by predicting the local stresses, however, executing FEM in a patient customized fashion would be costly, time consuming, and technically difficult for surgeons to perform.

Previous experimental studies of cortical bone have investigated cortical bone fracture toughness and crack trajectory as a function of microstructural alignment of osteons [1,2] . The dependence of osteon orientation on screw pullout force and crack propagation trajectory during screw pullout has been demonstrated previously by Feerick and McGarry (2012) [3] . The alternate failure modes for longitudinal and transverse screw pullout observed in the latter study are shown in Figure 1. Using an isotropic damage criterion with crack growth was simulated using an element deletion technique. An explicit representation of cortical bone microstructure was required to replicate experimental observations. The use of such a computational scheme for 3D macro-scale applications is not viable given the requirement of explicit representation of the microstructure. Other computational studies of cortical bone have also developed geometric representations of the microstructure of cortical bone to simulate the fracture and establish crack trajectories [4] . Again, upscaling these detailed microstructural geometries in 3D macroscale simulations of fracture would currently be computationally unfeasible.

Proximal pedicle screw pullout is a common clinical occurrence for long fusion constructs. Recently, novel spinal hardware and surgical techniques have been in development to alleviate this complication [1]. However, there is currently no biomechanical model to simulate this mode of failure in vitro to adequately evaluate these strategies in a rigorous laboratory setting. Standard pure moment loading and range of motion testing are not equipped to reproduce this failure modality [2] and a vast majority of such outcomes have merely been observed clinically [3]. It is hypothesized that a combination of anterior-posterior (AP) shear and compressive force is required to induce screw pullout with clinically similar fatigue patterns. The goal of this preliminary study is the development of such a biomechanical model to simulate clinically observed proximal pedicle screw failure.

Cervical corpectomy is a procedure most commonly indicated for resection of metastatic disease in the vertebra, access to the spinal cord tumors and inflammatory or infectious lesions.[1] Posterior occipitocervical instrumentation with rods and lateral mass screws has been shown to be a rigid fixation technique in this region [2] and, small diameter rods are thought to be lighter weight, less prominent and less likely to be associated with screw pullout.[3] Still, deformity imposed upon small diameter rods, by the weight of the head, the cervical spine, and spinal ligaments has yet to be quantified. Anecdotal observations show that the rods lose their lordotic curvature and patients fuse in a kyphotic curve making daily living more difficult. The goal of this study is to examine the mechanical behavior of these rods in situ under physiologic loading conditions reflective of activies of daily living.

A spinal instrument with pedicle screws is one of the most effective surgical techniques for correction of spinal deformity. However, when excessive mechanical loads are applied on the implant upon the fixation, there is a concern for back out of the screw in the cases with poor bone condition such as osteoporosis. For that reason, the mechanical loads at the bone-screw interface should be analyzed for safe and successful correction.