Initial Fixation Stability Research Articles

Fixation response of two cementless tibial implants under static and fatigue compression loading

The short- and long-term success of tibial cementless implants depend on the initial fixation stability often provided by posts and screws. Excessive relative interface motions are known to inhibit bone ingrowth and, hence, biologic fixation. In the present study, the stability of two cementless tibial implants under static and cyclic loading was studied. An apparatus was developed to perform compression tests on tibial components fixed by screws or posts onto resurfaced tibiae. Motion transducers were placed to record liftoff and subsidence displacements. A load of 1000 N was eccentrically applied on the medial plateau via the femoral component. The response was measured under static and repetitive loading up to 4000 cycles at 1 Hz. Experimental results showed that the screw fixation yielded the stiffer response and, hence, provided greater initial stability. The tibial component fixed with posts demonstrated larger lift off displacements. Comparison of measured results of static tests before and after fatigue indicated that the load-displacement behavior was not significantly affected by the cyclic fatigue loading under 1000 N up to 4000 cycles at 1 Hz. Nevertheless, much larger permanent displacements were recorded for the post fixation systems. In contrast to screw fixation, posts fixation systems demonstrated marked deterioration under larger compression loads of approximately 2000 N.

Interfragmentary motion in tibial osteotomies stabilized with ring fixators.

Relative movement of bone fragments affects healing processes. In vivo data exist for patients with reduced transverse fractures only. The gap movements that occur under more complex conditions such as in tibial osteotomies, however, are unknown. The goal of this study was to determine the initial gap movements in tibial correction osteotomies, to monitor movements during the early healing period, and to determine the suitability of initial fixation stability in relation to daily activities. The interfragmentary movements were measured in six patients with correction osteotomies stabilized by Ilizarov ring fixator constructs until union or until conversion to internal fixation. Consolidation was determined by clinical evaluation and standard radiographic techniques. Co-contraction led to gap movements comparable with level walking or standing. Shear generally exceeded axial compression. Although ground reactions and fixation stiffness were comparable with those reported for reduced fractures, movement magnitudes generally were larger than 2 mm. The shear movement component reflected the nature of the mechanical conditions at the bone gap. In a direct comparison with data from animal experiments, the local mechanical environment at the bone gap seemed unstable rather than overly stable. The method introduced in this study opens the perspective of adjusting osteosynthesis stability to the specific needs of each patient.

Composite hip prosthesis design. I. Analysis

An investigation was performed to study the mechanical performance of fiber-reinforced composite hip prostheses in a femur. The main objective of the study was to evaluate the effect of fiber orientation of a composite femoral implant on the response of the surrounding femoral bone. A three-dimensional finite element analysis was developed for analyzing a composite implant in the femur. A three-dimensional composite element was proposed to take into account ply drop-off due to a change of cross-section of the composite implant. The element could accommodate multidirectional layers and tapered composites. The material properties of the composite were treated as anisotropic and inhomogeneous while the properties of femoral bone were treated as anisotropic and homogenous. All the materials were assumed to behave linear-elastically. The thermoplastic graphite/PEEK material system was selected for the study. In this presentation, as the first part of the study, the development of the finite element analysis will be described. Numerical calculations were generated and compared with existing data and numerical results available from studies related to metal hip prostheses in the literature. Experiments on the composite hip implants were also conducted for further verification of the analysis and the computer simulations. In Part II, using the finite element code, an extensive study was performed to evaluate the stress/strain distributions, micromotions, and strain-energy density of the surrounding femoral bone, which have been related to initial fixation and long-term stability of the prosthesis in a femoral bone. Numerous fiber orientations were studied, and results of the calculations were compared with those generated by the prosthesis made of cobalt alloy and titanium alloy. © 1998 John Wiley & Sons, Inc. J Biomed Mater Res, 39, 92–101, 1998.

Composite hip prosthesis design. II. Simulation.

An investigation was performed to study the mechanical performance of fiber-reinforced composite hip prostheses. In Part I of the study, a three-dimensional finite element code was developed for analyzing a composite hip prosthesis in a femur. The material properties of the composite were treated as anisotropic and inhomogeneous while the properties of the femoral bone were treated as anisotropic and homogeneous. All the materials were assumed to behave linear-elastically. Thermoplastic graphite/PEEK material was selected for the study. No slippage was assumed at the interface between the implant and the surrounding femoral bone. In Part II, numerical simulations were performed using the code to study the performance of a composite prosthesis in the femur. The stress/strain distributions, micromotions, and strain energy density of the surrounding femoral bone were evaluated and found to be related to initial fixation and long-term stability of the prosthesis in the femur. Numerous fiber orientations were studied, and the results of the calculations were compared with those generated from a prosthesis made of cobalt chrome and Ti-6Al-4V titanium alloys. Based on the analysis, it was shown that compared to conventional metallic implants more favorable stresses and deformations could be generated in the femur using composite implants. In addition, by changing fiber orientations according to femoral loads, a composite implant could be designed specifically for the left or the right femur.

Composite hip prosthesis design. I. Analysis

An investigation was performed to study the mechanical performance of fiber-reinforced composite hip prostheses in a femur. The main objective of the study was to evaluate the effect of fiber orientation of a composite femoral implant on the response of the surrounding femoral bone. A three-dimensional finite element analysis was developed for analyzing a composite implant in the femur. A three-dimensional composite element was proposed to take into account ply drop-off due to a change of cross-section of the composite implant. The element could accommodate multidirectional layers and tapered composites. The material properties of the composite were treated as anisotropic and inhomogeneous while the properties of femoral bone were treated as anisotropic and homogenous. All the materials were assumed to behave linear-elastically. The thermoplastic graphite/PEEK material system was selected for the study. In this presentation, as the first part of the study, the development of the finite element analysis will be described. Numerical calculations were generated and compared with existing data and numerical results available from studies related to metal hip prostheses in the literature. Experiments on the composite hip implants were also conducted for further verification of the analysis and the computer simulations. In Part II, using the finite element code, an extensive study was performed to evaluate the stress/strain distributions, micromotions, and strain-energy density of the surrounding femoral bone, which have been related to initial fixation and long-term stability of the prosthesis in a femoral bone. Numerous fiber orientations were studied, and results of the calculations were compared with those generated by the prosthesis made of cobalt alloy and titanium alloy.

Motion observed across maxillary continuity defects stabilized with plates and screws.

The stability of fixation obtained with internal fixation devices (plates and screws) under functional loading of maxillary continuity defects was studied using fresh cadaver skulls. Micromotion (deformation) was recorded in the model across the level of the LeFort I osteotomies even when fixation was thought to be grossly stable. Deformation in this biologically static model proved to be mostly elastic in nature, and the amount of deformation in each skull appeared to be predictable from the average stability of all screws used in each plate. Such motion in viable bone could lead to permanent deformation due to resorption and remodeling of bone around the screws if initial overall fixation stability is tenuous. The results suggest that the term "rigid" is inappropriately used to describe plate and screw fixation of maxillary continuity defects.

Graft selection, placement, fixation, and tensioning for anterior cruciate ligament reconstruction

The ideal graft for replacement of the anterior cruciate ligament (ACL) is one that retains at least an equivalent level of normal ACL strength, allows for secure fixation, enables unrestricted rehabilitation, and has no graft harvest morbidity. The choices for ACL grafts are both biologic and synthetic. Among the biologic choices are autografts and allografts. One must decide on the type of biologic tissue, whether patellar tendon, hamstring tendon, iliotibial band, or Achilles tendon. In addition to selecting a graft, the ACL substitute must be correctly placed with secure fixation and under appropriate tension. Correct placement of a graft requires an accurate understanding of the normal anatomic attachment sites of the ACL, both on the tibia and the femur. Ideal fixation will allow secure initial fixation and sufficient stability until the native bone and/or tissue heals. This fixation must also be strong enough to allow the recommended postoperative rehabilitation. The tension of the graft must be sufficient to achieve stability but not so excessive that it captures the joint. ACL reconstruction depends on the surgeon understanding these options and possessing the necessary skills to perform the procedure. The goals of the surgery—restoring normal knee stability and allowing expedient recovery—depend on the surgeon knowing the ACL graft choices, the proper placement within the joint, the ideal fixation, and the proper tension.

The Effect of Screws and Pegs on the Initial Fixation Stability of an Uncemented Unicondylar Knee Replacement

Two uncemented unicompartmental tibial components were examined for initial fixation stability. A conventional design that employed a single posteriorly angled peg was compared with a new design that was held in place by cancellous bone screws. The components were implanted into the medial condyles of 12 preserved human tibiae, and a cyclic load was first applied anteromedially and then posteromedially. The screwed implants failed at significantly higher loads (1634.8 +/- 121.6 N, mean +/- standard error of the mean) than the pegged implants (1103.3 +/- 152.0 N). On application of a 19.6-N preload, the screwed implants moved significantly less than the pegged implants. Although the differences in micromotion and subsidence were not always significant, there were definite trends. The screwed implants had much lower levels of temporary and permanent displacement compared with the pegged implants for all load levels from the initial load of 245.2 N up to and including the failure load. When the motion that resulted from moving the load from the anterior position to the posterior position was examined, the screwed implant's average total motion was less than 10 microns compared with almost 135 microns for the pegged implant after the 245.2-N load cycle. For the cycle before failure, the screwed implant's average motion increased to less than 29 microns, whereas the pegged implant's average total motion was almost 354 microns. From this information it appears clear that screws provide better initial fixation stability than angled pegs for uncemented unicondylar tibial components.