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
A large number of total hip arthroplasties (THA) are performed each year, of which 60 % use cementless femoral fixation. This means that the implant is press-fitted in the bone by hammer blows. The initial fixation is one of the most important factors for a long lasting fixation [Gheduzzi 2007]. It is not easy to obtain the point of optimal initial fixation, because excessively press-fitting the implant by the hammer blows can cause peak stresses resulting in femoral fracture. In order to reduce these peak stresses during reaming, IMT Integral Medizintechnik (Luzern, Switzerland) designed the Woodpecker, a pneumatic reaming device using a vibrating tool. This study explores the feasibility of using this Woodpecker for implant insertion and detection of optimal fixation by analyzing the vibrational response of the implant and Woodpecker. The press-fit of the implant is quantified by measuring the strain in the cortical bone surrounding the implant. An in vitro study is presented. Two replica femur models (Sawbones Europe AB, Malmo Sweden) were used in this study. One of the femur models was instrumented with three rectangular strain gauge rosettes (Micro-Measurements, Raleigh, USA). The rosettes were placed medially, posteriorly and anteriorly on the proximal femur. Five paired implant insertions were performed on both bone models, alternating between standard hammer blow insertions and using the Woodpecker. The vibrational response was measured during the insertion process, at the implant and Woodpecker side using two shock accelerometers (PCB Piezotronics, Depew, NY, USA). The endpoint of insertion was defined as the point when the static strain stopped increasing. Significant trends were observed in the bandpower feature that was calculated from the vibrational spectrum at the implant side during the Woodpecker insertion. The bandpower is defined as the percentage power of the spectrum in the band 0–1000 Hz. Peak stress values calculated from the strain measurement during the insertion showed to be significantly (p < 0.05) lower at two locations using the Woodpecker compared to the hammer blows at the same level of static strain. However, the final static strain at the endpoint of insertion was approximately a factor two lower using the Woodpecker compared to the hammer. A decreasing trend was observed in the bandpower feature, followed by a stagnation. This point of stagnation was correlated with the stagnation of the periprosthetic stress in the bone measured by the strain gages. The behavior of this bandpower feature shows the possibility of using vibrational measurements during insertion to assess the endpoint of insertion. However it needs to be taken into account that it was not possible to reach the same level of static strain using the Woodpecker as with the hammer insertion. This could mean that either extra hammer blows or a more powerful pneumatic device could be needed for proper implant insertion.
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
Cementless femoral hip stems crucially depend on the initial stability to ensure a long survival of the prosthesis. There is only a small margin between obtaining the optimal press fit and a femoral fracture. The incidence of an intraoperative fracture is reported to be as high as 30% for revision surgery. The aim of this study is to assess what information is contained in the acoustic sound produced by the insertion hammer blows and explore whether this information can be used to assess optimal seating and warn for impeding fractures. Acoustic measurements of the stem insertion hammer blows were taken intra-operatively during 7 cementless primary (Wright Profemur Primary) and 2 cementless revision surgeries (Wright Profemur R Revision). All surgeries were carried out by the same experienced surgeon. The sound was recorded using 6 microphones (PCB 130E2), mounted at a distance of approximately 1 meter from the surgical theater. The 7 primary implants were inserted without complication, 1 revision stem induced a fracture distally during the insertion process. Two surgeons were asked to listen independently to the acoustic sounds post-surgery and to label the hits in the signal they would associate with either a fully fixated implant or with a fracture sound. For 3 out of 7 primary measurements the data was labeled the same by the two surgeons, 4 were labeled differently or undecided and both indicated several hits that would be associated with fracture for the fractured revision case. The acquired time signals were processed using a number of time and frequency domain processing techniques.Introduction
Materials and Methods
In total hip replacement (THR), the initial fixation of the femoral stem has a critical influence on its long term stability. Objective intra-operative assessment of primary stability is a challenge, surgeons having to rely mainly on their clinical experience. Excessive press-fitting of the stem can cause intra-operative fractures in up to 30% of revision cases. In a previous study we demonstrated the feasibility and validity of a vibrational technique for the assessment of the femur-stem fixation in vitro. In this in vivo study the vibration analysis was applied for the per-operative assessment of stem fixation in 30 THR patients who obtained an intra-operatively manufactured, hydroxyapatite coated, cementless prosthesis. The surgeon inserted the stem through repetitive controlled hammer blows. After each blow, the frequency response function (FRF) of the stem-bone structure was measured directly on the prosthesis neck in the range 0–10 kHz. The hammering was stopped when the FRF graph did not change anymore. Extra blows would not improve the stability but would increase the fracture risk. In 26 out of 30 cases (86.7%), the correlation coefficient between the last two FRFs was above 0.99 when the insertion was stopped. In four cases, when the surgeon decided to stop the insertion because of suspected bone fragility, the final correlation coefficient attained lower values. During the insertion of a cementless prosthesis, the changes of boundary conditions and implant stability between subsequent stages are reflected by the FRF evolution. The higher resonance frequencies are more sensitive to the stability change. The correlation between successive FRFs can be used as a criterion for the detection of the insertion endpoint. Moreover, the FRF analysis can be used to detect dangerous situations during surgery like stem blockage and fracture risk. This study should be completed and validated by a post-operative follow-up of the patients.
