Comparison of Test Setups for the Experimental Evaluation of the Primary Fixation Stability of Acetabular Cups

Abstract

Sufficient primary fixation stability is the basis for the osseointegration of cementless acetabular cups. Several test methods have been established for determining the tilting moment of acetabular press-fit cups, which is a measure for their primary fixation stability. The central aim of this experimental study was to show the differences between the commonly used lever-out test method (Method 1) and the edge-load test method (Method 2) in which the cup insert is axially loaded (1 kN) during the tilting process with respect to the parameters, tilting moment, and interface stiffness. Therefore, using a biomechanical cup block model, a press-fit cup design with a macro-structured surface was pushed into three cavity types (intact, moderate superior defect, and two-point-pinching cavity) made of 15 pcf and 30 pcf polyurethane foam blocks (n = 3 per cavity and foam density combination), respectively. Subsequently, the acetabular cup was disassembled from the three artificial bone cavities using the lever-out and the edge-load test method. Tilting moments determined with Method 1 ranged from 2.72 ± 0.29 Nm to 49.08 ± 1.50 Nm, and with Method 2, they ranged from 41.40 ± 1.05 Nm to 112.86 ± 5.29 Nm. In Method 2, larger areas of abrasion were observed in the artificial bone cavity compared to Method 1. This indicates increased shear forces at the implant–bone interface in the former method. In conclusion, Method 1 simulates the technique used by orthopedic surgeons to assess the correct fit of the trial cup, while Method 2 simulates the tilting of the cup in the acetabular bone cavity under in situ loading with the hip resultant force.

Experimental evaluation of the primary fixation stability of uncemented ceramic hip resurfacing implants

“…The push-on forces applied to reach a predefined position increased with increasing density of the artificial bone material, as it is observed for acetabular cups. 22,28,29 Moreover, the push-on forces increased with enhanced diametrical press-fit. The highest push-on force observed was in a similar range to the assembly forces that are used in standard THA when the ball head is assembled to the stem taper.…”

“…In previous in vivo and experimental studies, the frictional moments of various bearing materials and sizes were determined. 19,20,33–35 Riva et al analyzed frictional moments of CoC bearings with a diameter of 57 mm with loading according to ISO 14242-3, which ranged from 6.5 to 6.9 Nm. 20 Therefore, the frictional moments determined by Riva et al 20 already reached up to 81% of the turn-off moments determined in the present study, where the smaller-sized HRA was inserted in a low-density foam with the smaller press-fit.…”

Differences in acoustic parameters of hammering sounds between successful and unsuccessful initial cementless cup press-fit fixation in total hip arthroplasty

“…5,6 Insufficiently forceful hammering may result in inadequate initial fixation, leading to early cup loosening, while excessive hammering may cause intraoperative acetabular fracture. Several mechanical tests are available to confirm the cup stability intraoperatively, including the torque test, 4,7 lever-out test, [8][9][10] and hands-free test. Although these tests are useful, they are invasive and potentially destructive to surrounding tissue.…”

Novel Load Systems for In Vitro Testing of Biomaterials and Medical Devices

“…In the Special Issue, seven papers were published which explore different contexts in which the design of novel loading systems constituted the originality of the research. They range from the characterization of the mechanical behavior of biomaterials [ 1 , 2 ] and of biological tissues [ 3 ], to the verification of the structural behavior of more or less complex medical devices [ 4 , 5 ], from the design of tests for verifying the stability of articular prosthetic components once coupled to the bone [ 6 ], to the simulation of the erosive effect of surgical meshes on the biological soft tissues with which they interface [ 7 ].…”

“…Several test methods have been established for determining the tilting moment of acetabular press-fit cups, which is a measure for their primary fixation stability. The central aim of the experimental study explained in [ 6 ] is to show the differences between the commonly used lever-out test method and the edge-load test method in which the cup insert is axially loaded during the tilting process with respect to the parameters, tilting moment, and interface stiffness. At last, the research shown in [ 7 ] deals with the phenomenon of meshes erosion whereby soft tissue becomes damaged as a result of contact with implants made from surgical mesh, a fabric-like material consisting of fibers of polypropylene or other polymers.…”