Christine A. Buckley

Mechanical Stability of Novel Highly Porous Metal Acetabular Components in Revision Total Hip Arthroplasty

Highly porous metal acetabular components have emerged for revision hip arthroplasty. However, superior mechanical stability over traditional cementless components has not been demonstrated. Three different cementless acetabular components, including 2 highly porous tantalum designs, were inserted into hemipelvis specimens with a superolateral defect. Mechanical testing was performed to failure using a servohydraulic testing machine. The porous tantalum designs exhibited superior stability over the traditional cementless implant (P < .05). There was no difference in mechanical stability between the rigid modular tantalum shell and the more flexible revision tantalum shell (P > .46). In acetabular revision, highly porous tantalum acetabular components provide superior mechanical stability. However, these results suggest that improved frictional resistance is a more important design feature over implant flexibility with this particular implant.

Effects of Femoral Component Size on Proximal Tibial Strain With Anatomic Graduated Components Total Knee Arthroplasty

Survivorship in total knee arthroplasty (TKA) is multifactorial and dependent upon alignment, ligament balance, patient characteristics, and implant factors. The contribution of each factor leading to implant loosening is not well known. This study defined the effect of femoral component sizing relative to tibial size on loading patterns in the proximal tibia. Changes in strain were measured in tibiae implanted with appropriately sized metal-backed tibial components loaded with 2 sizes of femoral components. Significant increases of shear strain up to 126% were measured in peripheral regions of the tibia when loaded with a larger vs a smaller femoral component. Increased peripheral loading in the proximal tibia could predispose to a higher risk of cancellous overload and failure. Limiting stress concentrations in the periphery of the proximal tibia by considering sizing relationships between femoral and tibial components may decrease osseous strains and the likelihood of bony overload in TKA.

Metal Backing Significantly Decreases Tibial Strains in a Medial Unicompartmental Knee Arthroplasty Model

Clinical success of unicompartmental knee arthroplasty (UKA) is on the rise and is dependent on multiple patient, implant, and surgical factors. Tibial subsidence has been clinically reported as a cause of failure in UKA with an all-polyethylene tibial design in the absence of metal backing, yet the role of metal backing UKA tibial components on tibial loading is not fully understood. In this study, composite tibiae were implanted with medial all-polyethylene fixed-bearing or metal-backed UKA tibial components and a 1.5-kN load applied in 3 different contact positions simulating femoral translation during gait. All-polyethylene tibial components exhibited significantly higher strain measurements in each femoral position. This study demonstrates the role that metal backing plays on generating an even loading distribution while diminishing the development localized regions of excessive loading across the medial tibial cortex.

The effects of bone resection depth and malalignment on strain in the proximal tibia after total knee arthroplasty.

The clinical significance of tibial resection depth in total knee arthroplasty (TKA) is not clearly understood. The purpose of this study was to quantify the effect of tibial resection depth in TKA on tibial loading. Tibiae were coated with a photoelastic resin enabling full-field dynamic shear strain quantification in the tibial metaphysis during TKA loading. A standard resection level (5 mm) was compared to a resection level 15 mm distal to the joint line. Both had appropriate-sized tibial components. With 15 mm of tibial resection, strains increased up to 281% in the proximal and peripheral regions of the tibia during neutral loading and up to 315% anteriorly and 197% peripherally during varus loading. Distal resection levels result in significant smaller component size and relatively posterior and peripheral displacement of the implant. Changes in loading patterns in specimens with increased tibial resection depths have not previously been described.

High Initial Stability in Porous Titanium Acetabular Cups: A Biomechanical Study

Initial stability with limited micromotion in uncemented total hip arthroplasty acetabular components is essential for bony attachment and long-term biomechanical fixation. This study compared porous titanium fixation surfaces to clinically established, plasma-sprayed designs in terms of interface stability and required seating force. Porous plasma-sprayed modular and metal-on-metal (MOM) cups were compared to a modular, porous titanium designs. Cups were implanted into polyurethane blocks with1-mm interference fit and subsequently edge loaded to failure. Porous titanium cups exhibited 23% to 65% improvement in initial stability when compared to plasma-sprayed cup designs (P=.01): a clinically significant increase, based on experience and prior literature. The results of this study indicate increased interface stability in porous titanium-coated cups without significantly increasing the necessary force and energy required for full seating.

Acetabular Cup Stiffness and Implant Orientation Change Acetabular Loading Patterns

Acetabular cup orientation has been shown to influence dislocation, impingement, edge loading, contact stress, and polyethylene wear in total hip arthroplasty. Acetabular implant stiffness has been suggested as a factor in pelvic stress shielding and osseous integration. This study was designed to examine the combined effects of acetabular cup orientation and stiffness and on pelvic osseous loading. Four implant designs of varying stiffness were implanted into a composite hemipelvis in 35° or 50° of abduction. Specimens were dynamically loaded to simulate gait and pelvic strains were quantified with a grid of rosette strain gages and digital image correlation techniques. Changes in the joint reaction force orientation significantly altered mean acetabular bone strain values up to 67%. Increased cup abduction resulted in a 12% increase along the medial acetabular wall and an 18% decrease in strain in inferior lateral regions. Imbalanced loading distributions were observed with the stiffer components, resulting in higher, more variable, and localized surface strains. This study illustrates the effects of cup stiffness, gait, and implant orientation on loading distributions across the implanted pelvis.

A Comparison in Proximal Tibial Strain Between Metal-Backed and All-Polyethylene Anatomic Graduated Component Total Knee Arthroplasty Tibial Components

Loading in total knee arthroplasty (TKA) is multifactorial and dependent on alignment, ligament balance, patient, and implant factors. Abnormal loading has been linked to clinical failure; however, the respective contribution of each factor to failure is not well known. This study defined the effect of metal backing on loading patterns in the proximal tibia. Composite tibiae were implanted with metal-backed and all-polyethylene Anatomic Graduated Component TKA tibial components (Biomet, Inc, Warsaw, Ind) and coated with photoelastic material allowing full-field dynamic strain quantification. In simulated varus loading distributions, significant increases in measured strain were observed ranging from 40% to 587% for all-polyethylene vs metal-backed tibial components. Higher observed strains in the proximal tibia observed with all-polyethylene tibial components could possibly explain increased clinical failure rates observed with this TKA design.

Does Ischial Screw Fixation Improve Mechanical Stability in Revision Total Hip Arthroplasty

Ischial screw fixation, albeit technically challenging, is postulated to provide additional mechanical stability in revision total hip arthroplasty (THA). Hemipelvis specimens were prepared to simulate revision THA, and an acetabular component with supplemental screw fixation was implanted. Three configurations were tested: 2 dome screws alone, 2 dome screws plus an additional screw within the dome, and 2 dome screws plus an ischial screw. Force displacement data were acquired during mechanical testing. An increase in mechanical stability was observed in acetabular components with supplemental screw fixation into either the posterior column or ischium (P≤.031) compared to isolated dome fixation. In addition, supplemental ischial screw fixation may provide a modest advantage over a screw placed posteroinferiorly within the acetabular dome during revision THA.

Shell design and reaming technique affect deformation in mobile-bearing total hip arthroplasty acetabular components:

Press-fit acetabular components are susceptible to rim deformation. The inherent variability within acetabular reaming techniques may generate increased press-fit and, subsequently, additional component deformation. The purpose of this study was to analyze the insertion and deformation characteristics of acetabular components designed for dual-mobility systems based on component design, size, and reaming technique. Shell deformation was quantified in a validated worst-case scenario foam pinch model. Thin-walled, one-piece, and modular dual-mobility shells of varying size were implanted in under- and over-reamed cavities with insertion force measured and shell deformation assessed using digital image correlation. Increased shell size resulted in larger rim deformation in one-piece components, with a reduction in press-fit by 1 mm resulting in up to 48% reduction in insertion forces and between 23% and 51% reduction in shell deformation. Lower insertion forces and deformations were observed in modular components. Variability in acetabular reaming plays a significant role in the ease of implantation and component deformation in total hip arthroplasty. Modular components are less susceptible to deformation than thin-walled monoblock shells. Care should be taken to avoid excessive under-reaming, particularly in the scenario of large shell size and high-density patient bone stock.