Properties of composite specimens of old and new bone cement.

The most common cause of failure of a total hip replacement is aseptic loosening of an implant. In a number of cases, the cement-bone interface of at least one component is not compromised. In cases of aseptic cup loosening, removal of a well-fixed femoral stem may be undertaken to facilitate exposure of the acetabulum for cup revision, and the surgeon may choose to leave the functional cement-bone interfaces in the femur undisturbed. After cup revision, new cement is pressurized within the old cement mantle and a stem is cemented into this 'old-new cement' composite. Retaining the old cement mantle is an attractive option as it reduces the duration of surgery, minimizes bleeding, and preserves the bone stock. Excellent results have been shown with this technique of 'in-cement femoral revision' using a double-tapered polished stem. While considerable literature is available on the short- and long-term properties of PMMA bone cement, very little is known about the mechanical properties of old-new composite cement specimens where the old cement is more than a few days old. This paper tests the properties of such old-new composite specimens where the 'old' cement is aged between 3.3 and 17.7 years, better reflecting clinical situations.

Torsional stability of a polished, collarless, tapered total replacement hip joint stem under vertical load: an in vitro investigation.

The torsional stability of a polished, collarless, tapered total replacement hip joint stem has been investigated. It was believed that such a stem would show increasing torsional stability as the vertical component of load on the stem increased. The aim of the study was to examine this hypothesis by testing a number of specimens (including Exeter stems) with either a matt or polished finish, under increasing vertical load, measuring the torsional resistance of a specimen-cement construct within an outer constrained cement shell. It was concluded that stems with a polished, collarless, tapered shape showed increased torsional stability with increasing vertical load, while stems with a matt finish or a collar did not.

The effect of the Rim Cutter on cement pressurization and penetration on cemented acetabular fixation in total hip arthroplasty: an in vitro study.

The Rim Cutter (Stryker Orthopedics, Mahwah, New Jersey) is a tool designed to cut a ledge inside the rim of the acetabulum, onto which a precisely trimmed, cemented, flanged cup can be fitted. The aim was to investigate the effect of the Rim Cutter on the intra-acetabular cement mantle pressure and the depth of cement penetration during cup insertion. The study had two parts. In the first part, hemi-pelvis models were fitted with pressure sensors. Pressure in the acetabulum was measured on insertion of a conventional cemented flanged cup with and without the use of a Rim Cutter to prepare the rim of the acetabulum. The second part assessed cement penetration when the same cups were inserted into a foam shell model. The shell was mounted in a jig and had holes drilled in it; the distance that cement penetrated into the holes was measured. A significant increase in cement pressure at the apex (p = 0.04) and the rim (p = 0.004) is seen when the Rim Cutter is used. Cement penetration in the Rim Cutter group was significantly increased at the rim of the acetabulum (p = 0.003). Insertion of a flanged cup after the acetabulum is prepared with the Rim Cutter leads to a significant increase in cement pressure and penetration during cup insertion in vitro when compared with conventional flanged cups.

Controlled plastic deformation for the fastening mechanism of an internal fixation device. The new Mennen 3 PeriPro plate.

The Mennen femur plate is a fixation device used for the treatment of femoral periprosthetic fractures. It features a novel fastening method where curved prongs are plastically deformed securing the implant to the bone. Although this "clamp-on" method has been successfully used to treat fractures of long bones, there are no literature data assessing the nature of the required plastic deformation. In the present study, the parameters influencing the performance of the prongs were identified and further explored using numerical modeling. The new Mennen 3 PeriPro plate is briefly discussed focusing on the new sculpted formation of the prongs. Their design was optimized to effectively control the magnitude and position of the required plastic deformation achieving enhanced anchorage on the fractured bone with minimum effort. The work presented contains all the necessary steps in analysing a clinical problem using finite elements and illustrates how effective use of simulation techniques can accurately predict and effectively control the required plastic deformation of a structure.

The mechanical properties of recovered PMMA bone cement: a preliminary study.

Samples of polymethylmethacrylate (PMMA) bone cement, used in the fixation of hip prostheses, have been recovered from 11 patients after in service life spans of between 15 and 24 years. Eighteen samples in total have been recovered from the acetabular and/or femoral cement. Samples were subjected to three point bending tests, their density, porosity and microhardness determined and all specimens were examined using EDX and X-ray techniques. Since the porosity of many of the samples is very high, the continuous matrix properties are inferred from the performance of individual specimens. No evidence has been found to suggest that the PMMA has deteriorated whilst in-vivo and the mechanical properties of the cement matrices appear to be comparable to freshly made PMMA.

Development of the mennen 3 peripro fixation plate for the treatment of periprosthetic fractures of the femur.

The Mennen femur plate is an internal fixation device used for the management of femoral perisprosthetic fractures, usually after total hip replacement surgery. The implant uses a number of curved prongs that embrace the fractured bone around its circumference without interfering with the stem of the prosthesis. Although the device has been used with considerable clinical success since its first introduction, a number of negative clinical results have been reported in the literature. The failure modes of the device are described and an evaluation of its performance is briefly presented. Based on this assessment as well as comments in the open literature, modifications in the design of the device have been implemented. The new Mennen 3 PeriPro plate is presented, with all the necessary data for a coherent explanation of its improved characteristics as defined using numerical simulations and experimental tests. The new device has all the beneficial features of the previous plate with improved structural performance and fatigue life and new sculpted formation of the prongs, providing a simple implantation technique with maximum gripping and minimum effort from the surgeon. The unique mode of fixation has been further improved, providing ample anchorage on the fracture bone without compromising its biomechanical integrity. By combining the device with a cable system, the spectrum of applications will be further expanded, enabling the surgeon to treat a broader range of fracture patterns.

Assessment of wear on the cones of modular stainless steel Exeter hip stems.

The wear on the stem cones of retrieved Exeter Universal hip stems has been assessed using scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX) and surface profilometry. The in-service life of these prosthetic stems varied, up to a maximum of 7 years. A combination of SEM, EDX and visual assessment indicates that the stem cones have not suffered from any corrosion. SEM scans indicate that damage to stem cones (excluding extraction and post-removal damage) can be categorised into insertion marks and fretting marks. In some cases there are signs of material being deposited on the cone surface. Surface profilometry suggests that the levels of debris generation at the cone/internal head interface are very low relative to those that are likely to be associated with head articulation against the acetabular cup. A total of 20 stem cones underwent SEM scans. From these, 10 subsequently have undergone surface profilometry along with the corresponding internal head surfaces. There is a good correlation between surface roughness measured by surface profilometry and the topography observed in the SEM images. The surface roughness of each stem cone is similar to that of the corresponding internal head surface.

In vivo surface wear mechanisms of femoral components of cemented total hip arthroplasties: the influence of wear mechanism on clinical outcome.

The appearance and mechanism of femoral stem wear was studied in 172 retrieved femoral components, of which 74 stems had been stable in vivo. Macroscopic, microscopic, and nano-level scales of examination were used. Loss of stem surface in response to micromotion (wear) was found to affect 93% of stems. However, changes were frequently difficult to see with the naked eye, and in 19% of cases they would have been missed completely without the use of light microscopy. The surface finish of the prosthesis determined the mechanism of stem wear. Matte surfaces showed typical abrasive processes that also damage the cement, releasing particulate debris from the cement and metal surfaces. This may destabilize the stem within the cement. Polished stems showed a typical fretting appearance with retention of debris on the stem surface and without significant damage to the cement. These differences in wear mechanism between matte and polished stems have significant effects on stem function.

Stress relaxation modelling of polymethylmethacrylate bone cement.

This paper describes tests that were carried out to model the stress relaxation behaviour of polymethylmethacrylate (PMMA) bone cement. Stress relaxation of bone cement is believed to be a significant factor in the mechanism of load transfer in the femoral stem of a polished, collarless taper-fit replacement hip joints. It is therefore important that this condition and its implications are understood. Stress relaxation was carried out on PMMA samples of varying age in four-point bending configuration. It was shown that the samples stiffened with age and that the amount of stress relaxation reduced as the samples aged. The experimental results of the stress relaxation were accurately modelled on the double exponential of the Maxwell model so that long-term predictions of the stress condition could be made from short-term mechanical tests.

Factors affecting the mechanical and viscoelastic properties of acrylic bone cement.

The aim of this paper is to report a series of experiments investigating the factors that influence the viscoelastic properties of acrylic bone cement. The effects of the brand of cement, the length of time since mixing, temperature, the hydration of the cement, and the influence of fat and or blood in the environment on the creep and stress relaxation behavior of the cement have been studied in laboratory-prepared specimens in tension, compression and four point bending. Although there are significant differences in the viscoelastic behavior of some of the different brands of polymethylmethacrylate based cements, these differences are small by comparison with the major effects that can be exerted by the length of time since mixing and some environmental factors. These effects have important practical consequences, especially with regard to the ability of bench top and theoretical studies to predict reliably the mechanical and viscoelastic behavior of acrylic cement in vivo.