A biomechanical analysis of triangulation of anterior vertebral double-screw fixation

OBJECTIVE: This study tested the hypothesis that triangulation of two anterior vertebral screws without penetration of the cortex offers more resistance to pullout than two screws placed in parallel and penetrated. DESIGN: The pullout strength for two parallel or two triangulated anterior vertebral screws fixation, with a uni-cortical or bi-cortical purchase, were tested and compared to the strength of a single-screw fixation with a bi-cortical purchase. Four porcine spines (six months old) were used for biomechanical test and bone mineral density was measured for each specimen before testing. BACKGROUND: The potential hazards from penetration by anterior vertebral cortex screws including neurovascular and organs injuries are well documented. However, bi-cortical screw penetration is widely recognized as necessary for good anterior spinal stabilization. The authors are not aware of any biomechanical study on the anterior placement of triangulated vertebral screws without penetration and its effect on the fixation strength of anterior vertebral device remains unclear. METHODS: In this study five modes of screw fixations in lateral vertebral bodies were performed: Group A, triangulated screws with one screw penetration; Group B, triangulated screws without penetration; Group C, parallel penetrating screws; Group D, parallel nonpenetrating screws; and Group E, a single-screw with bi-cortical purchase. Biomechanical analysis with a material testing system machine was performed to determine the pull out strength of each configuration. RESULTS: The results showed that the pullout strength in the various double-screw fixation modes were statistically increased as compared to that of the single-screw with bi-cortical purchase mode. There existed statistical differences (P<0.05) between Groups A and B, Groups C and D and Groups D and E, respectively. However, no significant difference was found between Groups B and C (P=0.144). CONCLUSIONS: Based on the current data, triangulation of two anterior vertebral screws without penetration of the cortex (Group B) achieved pullout strengths similar to that of two-parallel double-cortical screws (Group C). The authors believe that this is an attractive alternative in anterior spinal instrumentation avoiding the potential risks of cortical penetration. However, in the event of pullout failure, the triangulation configuration will produce a more disastrous consequence. RELEVANCE: Triangulation of two anterior vertebral screws without penetration of the cortex achieve pullout strengths similar to that of two-parallel double-cortical screws. This is an attractive alternative in anterior spinal instrumentation that avoids the potential risks of cortical penetration.     

The degrees to which transtrochanteric rotational osteotomy moves the region of osteonecrotic femoral head out of the weight-bearing area as evaluated by computer simulation

BACKGROUND: Transtrochanteric rotational osteotomy is a technical demanding procedure. Currently, the pre-operative planning of the transtrochanteric rotational osteotomy is mostly based on X-ray images. The surgeons would need to reconstruct the three-dimensional structure of the femoral head and the necrosis in their mind. This study develops a simulation platform using computer models based on the computed tomography images of the femoral head to evaluate the degree to which transtrochanteric rotational osteotomy moves the region of osteonecrotic femoral head out of the weight-bearing area in stance and gait cycle conditions. Based on this simulation procedure, the surgeons would be better informed before the surgery and the indication can be carefully assessed. METHOD: A case with osteonecrosis involving 15% of the femoral head was recruited. Virtual models with the same size lesion but at different locations were devised. Computer models were created using SolidWorks 2000 CAD software. The area ratio of weight-bearing zone occupied by the necrotic lesion on two conditions, stance and gait cycle, were measured after surgery simulations. FINDINGS: For the specific case and virtual models devised in this study, computer simulation showed the following two findings: (1) The degrees needed to move the necrosis out of the weight-bearing zone in stance were less by anterior rotational osteotomy as compared to that of posterior rotational osteotomy. However, the necrotic region would still overlap with the weight-bearing area during gait cycle. (2) Because the degrees allowed for posterior rotation were less restricted than anterior rotation, posterior rotational osteotomies were often more effective to move the necrotic region out of the weight-bearing area during gait cycle. INTERPRETATION: The computer simulation platform by registering actual CT images is a useful tool to assess the direction and degrees needed for transtrochanteric rotational osteotomy. Although the results indicated that anterior rotational osteotomy was more effective to move the necrosis out of the weight-bearing zone in stance for models devised in this study, in circumstances where the necrotic region located at various locale, considering the limitation of anterior rotation inherited with the risk of vascular compromise, it might be more beneficial to perform posterior rotation osteotomy in taking account of gait cycle.     

Finite element analysis of the cervico-trochanteric stemless femoral prosthesis

OBJECTIVE: To investigate the biomechanical performance of a newly designed cervico-trochanteric stemless prosthesis by comparing the stress distribution with that of the traditional stem-type porous-coated anatomic prosthesis. DESIGN: Three-dimensional finite element models were created for the intact femur, cervico-trochanteric implanted femur and porous-coated anatomic implanted femur. The stress distributions on the femur and the implant were compared. The effects of using two or three screws fixation for the cervico-trochanteric implanted femur were also investigated. BACKGROUND: Local bone loss after implantation of traditional stem-type prostheses remains an unsolved problem during the long-term application of total hip replacement. The stress shielding effect and osteolysis were thought to be the two main factors that result in local bone loss after prosthesis implantation. In order to eliminate the mechanical and the biological causes of bone loss after total hip arthroplasty, a newly designed stemless femoral prosthesis was investigated. METHODS: Three-dimensional finite element models were created for the intact, cervico-trochanteric (with two or three fixation screws), and porous-coated anatomic implanted femora with the geometry of a standardized composite femur. Analysis was performed for a loading condition simulating the single-legged stance. The von Mises stress distributions of each model were analyzed and compared. RESULTS: The results can be summarized as follows: (1) Von Mises stress in the proximal, medial femur for the cervico-trochanteric implanted model was higher than that of the intact model and the porous-coated anatomic implanted model; (2) stress-shielding effect of the cervico-trochanteric models (with two or three fixation screws) were eliminated as compared with the porous-coated anatomic model; (3) no obvious difference in von Mises stress distribution for the cervico-trochanteric implanted model with two or three fixation screws. CONCLUSIONS: The cervico-trochanteric femoral prosthesis may reduce the stress-shielding effect of the proximal femur and achieve a more physiological stress distribution on the proximal femur than that of the porous-coated anatomic prosthesis. RELEVANCE: The new concept of cervico-trochanteric stemless prosthesis has proven to possess several advantages based on the current results, and may be an alternative for traditional stem-type prostheses in future clinical applications.     

Biomechanical optimization of different fixation modes for a proximal femoral L-osteotomy

Background  

Numerous proposed surgical techniques have had minimal success in managing greater trochanter overgrowth secondary to retarded growth of the femoral capital epiphysis. For reconstruction of residual hip deformities, a novel type of proximal femur L-osteotomy was performed with satisfactory results. Although the clinical outcome was good, the biomechanical characteristics of the femur after such an osteotomy have not been clearly elucidated. Therefore, this study presents a three dimensional finite element analysis designed to understand the mechanical characteristics of the femur after the L-osteotomy.     

The number of screws, bone quality, and friction coefficient affect acetabular cup stability

One of the major causes of loosening of cementless acetabular cup implants is insufficient initial stability. This study used three-dimensional finite element models of the pelvis and acetabular components to investigate the effects of the number of screws, bone quality, and friction coefficient of the acetabular cup on the initial stability under normal walking. A commercially available hemispherical acetabular cup with five screw holes was used as the default model. The stiffness of the pelvis and the friction coefficient of the cup were systematically varied, within a realistic range, to assess the initial stability of the acetabular cup. The simulations showed that the inserted screws provide only a localized reduction in the relative micromotion between the cup and pelvis therefore inserting several screw closed together might not be useful. Changes in the pelvic stiffness have a non-linear effect on the initial stability of acetabular cup and the subchondral bone provides good support for fixation of the cementless cup. The friction coefficient of the acetabular cup plays a limited role, comparing with the factor of bone quality, in resisting relative micromotion in the cup–pelvis interface.     

A novel dental implant abutment with micro-motion capability—Development and biomechanical evaluations

Objective

The aim of this study was to develop a novel dental implant abutment with a micro-motion mechanism that imitates the biomechanical behavior of the periodontal ligament, with the goal of increasing the long-term survival rate of dental implants.

Methods

Computer-aided design software was used to design a novel dental implant abutment with an internal resilient component with a micro-motion capability. The feasibility of the novel system was investigated via finite element analysis. Then, a prototype of the novel dental implant abutment was fabricated, and the mechanical behavior was evaluated.

Results

The results of the mechanical tests and finite element analysis confirmed that the novel dental implant abutment possessed the anticipated micro-motion capability. Furthermore, the nonlinear force–displacement behavior apparent in this micro-motion mechanism imitated the movement of a human tooth. The slope of the force–displacement curve of the novel abutment was approximately 38.5 N/mm before the 0.02-mm displacement and approximately 430 N/mm after the 0.03-mm displacement.

Significance

The novel dental implant abutment with a micro-motion mechanism actually imitated the biomechanical behavior of a natural tooth and provided resilient function, sealing, a non-separation mechanism, and ease-of-use.     

Determination of the augmentation effects of hyaluronic acid on different heel structures in amputated lower limbs of diabetic patients using ultrasound elastography

This study measured tissue properties of different anatomies of heels in amputated lower limbs of diabetic patients before and after hyaluronic acid (HA) or normal saline (NS) injections. Seven amputated lower limbs from six diabetic patients constituted the experimental group and one amputated lower limb from a diabetic patient served as the control. The limbs were placed in a fixation platform. A 5-12 MHz linear-array ultrasound transducer controlled by a stepping motor was used to load and unload tested heels. The loading-unloading velocity was 6 mm/s and the maximum loading stress was 178 kPa. Loading-unloading tests were performed before and after 1 mL HA injections into heels in the experimental group. The control limb underwent the same test before and after 1 mL NS injection. The unloaded thickness and Young's modulus of the macrochambers, microchambers and heel pads were determined before and after the interventions. The unloaded thickness of the macrochambers and the heel pad increased significantly (p = 0.012) after HA injection. The Young's modulus of the macrochambers decreased nonsignificantly after HA injections. Similar thickness and tissue stiffness changes were observed in the control limb. The baseline heel-pad energy dissipation ratio (EDR(hp)) was 81.3 ± 1.3% and decreased significantly (p = 0.012) to 73.1 ± 1.7% after HA injections. The EDR(hp) in the control increased after NS injection. Histologic examinations revealed localized HA accumulation in the macrochambers with an extension into the adjacent fibrous septa. Injection of HA can increase tissue thickness and enhance heel-pad tissue resilience.     

Selection of fixation devices in proximal femur rotational osteotomy: clinical complications and finite element analysis

OBJECTIVE: To compare the postoperative stability between large cancellous screws fixation and dynamic hip screws fixation in transtrochanteric rotational osteotomy. DESIGN: Finite element analysis was designed to validate the clinical outcomes of fixation failure in transtrochanteric rotational osteotomy. BACKGROUND: From 1997 to 2000, transtrochanteric rotational osteotomy had been done in 20 osteonecrotic hips. Fixation with large cancellous screws as suggested by Sugioka had been used in the first five hips. Unfortunately, fixation failures were encountered in all of the five cases. The fixation mode was then shifted to a combination of plate and screws, no fixation failure had been observed postoperatively. A finite element analysis was designed to investigate the causes of fixation failure. METHODS: Finite element models for four dynamic hip screw instrumented femora and four large screws instrumented femora simulating four different levels of bone cut were created. The von Mises stress distributions of each model were analyzed and compared for a loading condition simulating single-legged stance. RESULTS: Analysis results confirmed our clinical observation that fixation with screws had higher stress concentration on the proximal femur. Improper bone cut, especially those that are too distal from the intertrochanteric line, had higher risks of fixation failure. CONCLUSIONS: Transtrochanteric rotational osteotomy is a technically demanding procedure such that surgical principles should be abided carefully to avoid catastrophic complications. RELEVANCE: Finite element analysis results demonstrated that dynamic hip screw fixation provides better stability and prevents fixation failure that corresponds with the clinical observation.