Aseptic acetabular component failure rates have been reported to be similar or even slightly higher than femoral component failure. Obtaining proper initial stability by press fitting the cementless acetabular cup into an undersized cavity is crucial to allow for secondary osseous integration. However, finding the insertion endpoint that corresponds to an optimal initial stability is challenging. This in vitro study presents an alternative method that allows tracking the insertion progress of acetabular implants in a non-destructive, real-time manner. A simplified acetabular bone model was used for a series of insertion experiments. The bone model consisted of polyurethane solid foam blocks (Sawbones #1522-04 and #1522-05) into which a hemispherical cavity and cylindrical wall, representing the acetabular rim, were machined using a computer numerically controlled (CNC) milling machine (Haas Automation Inc., Oxnard, CA, USA). Fig. 1 depicts the bone model and setup used. A total of 10 insertions were carried out, 5 on a low density block, 5 on a high density block. The acetabular cups were press fitted into the bone models by succeeding hammer hits. The acceleration of the implant-insertor combination was measured using 2 shock accelerometers mounted on the insertor during the insertion process (PCB 350C03, PCB Depew, NY, USA). The force applied to the implant-insertor combination was also measured. 15 hammer hits were applied per insertion experiment. Two features were extracted from the acceleration time signal; total signal energy (E) and signal length (LS). Two features and one correlation measure were extracted from the acceleration frequency spectra; the relative signal power in the low frequency band (PL, from 500–2500Hz) and the signal power in the high frequency band (P Hf, from 4000–4800 Hz). The changes in the low frequency spectra (P Lf, from 500–2500 Hz) between two steps were tracked by calculating the Frequency Response Assurance Criterion (FRAC). Force features similar to the ones proposed by Mathieu et al., 2013 were obtained from the force time data. The convergence behavior of the features was tracked as insertion progressed.Introduction
Materials and Methods
Each year, a large number of total hip arthroplasties (THA) are performed, of which 60 % use cementless fixation. The initial fixation is one of the most important factors for a long lasting fixation [Gheduzzi 2007]. The point of optimal initial fixation, the endpoint of insertion, is not easy to achieve, as the margin between optimal fixation and a femoral fracture is small. Femoral fractures are caused by peak stresses induced during broaching or by the hammer blows when the implant is excessively press-fitted in the femur. In order to reduce the peak stresses during broaching, IMT Integral Medizintechnik (Luzern, Switzerland) designed the Woodpecker, a pneumatic broach that generates impulses at a frequency of 70 Hz. This study explores the feasibility of using the Woodpecker for implant insertion by measuring both the strain in the cortical bone and the vibrational response. An in vitro study is presented. A Profemur Gladiator modular stem (MicroPort Orthopedics Inc. Arlington, TN, USA) and two artificial femora (composite bone 4th generation #3403, Sawbones Europe AB, Malmö, Sweden) were used. One artificial femur was instrumented with three rectangular strain gauge rosettes (Micro-Measurements, Raleigh, NC, USA). The rosettes were placed medially, posteriorly and anteriorly proximally on the cortical bone. Five paired implant insertions were repeated on both artificial bones, alternating between standard hammering and Woodpecker insertions. During the insertion processes the vibrational response was measured at the implant and Woodpecker side (fig. 1) using two shock accelerometers (PCB Piezotronics, Depew, NY, USA). Frequency spectra were derived from the vibrational responses. The endpoint of insertion was defined as the point when the static strain stopped increasing during the insertion.Introduction
Material and Methods
The success of cementless total hip arthroplasty (THA), primary as well as for revision, largely depends on the initial stability of the femoral implant. In this respect, several studies have estimated that the micromotion at the bone-implant interface should not exceed 150µm (Jasty 1997, Viceconti 2000) in order to ensure optimal bonding between bone and implant. Therefore, evaluating the initial stability through micromotion measurements serves as a valid method towards reviewing implant design and its potential for uncemented THAs. In general, the methods used to measure the micromotion assume that the implant behaves as a rigid body. While this could be valid for some primary stems (Østbyhaug 2010), studies that support the same assumption related to revision implants were not found. The aim of this study is to assess the initial stability of a femoral revision stem, taking into account possible non-rigid behaviour of the implant. A new in vitro measuring method to determine the micromotion of femoral revision implants is presented. Both implant and bone induced displacements under cyclic load are measured locally. A Profemur R modular revision stem (MicroPort Orthopedics Inc. Arlington, TN, United States of America) and artificial femora (composite bone 4th generation #3403, Sawbones Europe AB, Malmö, Sweden) prepared by a surgeon were used. The micromotions were measured in proximal-distal, medial-lateral or anterior-posterior directions at four locations situated in two transverse planes, using pin and bushing combinations. At each measuring location an Ø8mm bushing was attached to the bone, and a concentric Ø3mm pin was attached to the implant [Fig.1 and 2]. A supporting structure used to hold either guiding bushings or linear variable displacement transducers (LVDT) is attached to the proximal part of the implant. The whole system was installed on a hydraulic force bench (PC160N, Schenck GmbH, Darmstadt, Germany) and 250 physiological loading cycles were applied [Fig.3].Introduction
Methods
The 3D interplay between femoral component placement on contact stresses and range of motion of hip resurfacing was investigated with a hip model. Pre- and post-operative contours of the bone geometry and the gluteus medius were obtained from grey-value CT-segmentations. The joint contact forces and stresses were simulated for variations in component placement during a normal gait. The effect of component placement on range of motion was determined with a collision model. The contact forces were not increased with optimal component placement due to the compensatory effect of the medialisation of the center of rotation. However, the total range of motion decreased by 33%. Accumulative displacements of the femoral and acetabular center of rotation could increase the contact stresses between 5–24%. Inclining and anteverting the socket further increased the contact stresses between 6–11%. Increased socket inclination and anteversion in combination with shortening of the neck were associated with extremely high contact stresses. The effect of femoral offset restoration on range of motion was significantly higher than the effect of socket positioning. In conclusion, displacement of the femoral center of rotation in the lateral direction is at least as important for failure of hip resurfacings as socket malpositioning.
Glenosphere disengagement can be a potential serious default in reverse shoulder arthroplasty [1]. To ensure a good clinical outcome, it is important for the surgeon to obtain an optimal assembly of the glenosphere - base plate system during surgery. However interpositioning of material particles (bone, soft tissue) between the contact surface of the glenosphere and the base plate and/or a misalignment of the glenosphere relative to the base plate can result in a suboptimal assembly of the glenosphere – base plate system [2]. This misalignment is typically caused by unwanted contact between the glenosphere and the scapula due to inadequate reaming. Both defects prevent the Morse taper from fully engaging, leading to a system configuration for which the assembly was not designed to be loaded in vivo. This study quantifies the influence these defects have on the relative movement between the glenosphere and metaglene. A biaxial test setup [Fig. 1] was developed to mechanically load the glenoidal assembly (base plate + glenosphere) of 5 Depuy® Delta Xtend 38 prostheses. The setup allows applying a cyclic loading pattern to the glenoidal component with a constant actuator load of 750 N. Each of the 5 samples was tested for 5000 cycles on 3 defects: an interpositioning of 150 µm thick (0.48 mm3) and two local underreaming defects, pushing one side of the glenosphere up 0.5 mm and 1 mm respectively, hence causing a misalignment. The relative movement was recorded using 4 Linear Variable Differential Transducers (LVDTs). The cycling frequency is 1 Hz.INTRODUCTION
MATERIALS AND METHODS
The bowing of the femur defines a curvature plane to which the proximal and distal femoral anatomic landmarks have a predictable interrelationship. This plane can be a helpful adjunct for computer navigation to define the pre-operative, non-diseased anatomy of the femur and more particularly the rotational alignment of the femoral component in total knee arthroplasty (TKA). There is very limited knowledge with regards to the sagittal curvature -or bowing- of the femur. It was our aim (1) to determine the most accurate assessment technique to define the femoral bowing, (2) to define the relationships of the curvature plane relative to proximal and distal anatomic landmarks and (3) to assess the position of femoral components of a TKA relative to the femoral bowing.Summary sentence
Background and aims
