Abstract

250 words max Long polished cemented femoral stems, such as the Exeter Hip Revision stem, are one option available to the revision hip arthroplasty surgeon. When proximal bone stock is compromised, distal fixation is often relied upon for stability of the femoral component. In such circumstances, torsional forces can result in debonding and loosening. This study compared the torsional behaviour of a cemented polished and featureless (plain) stem with cemented, polished stems featuring fins or flutes. Nine torsional tests were carried out on each of these three different stem designs. The finned stem construct was significantly stiffer than the fluted stem (mean 24.5 Nm/deg v 17.5 Nm/deg). The plain stem mean stiffness was less than the featured stems (13 Nm/deg), but wide variability lead to no statistically significant difference. The maximum torque of the finned (30.5 Nm) and fluted stems (29 Nm) was significantly higher than the plain stem (10.5 Nm); with no significance to the difference between the finned and fluted stems. Distal stem features may provide a more reliable and greater resistance to torque in polished, cemented revision hip stems. Finned stem features may also increase the stiffness of the construct. Consideration should thus be given to the incorporation of distal stem features in the design of revision hip stems.

Introduction: Inadequate cementation of the acetabular component in hip replacement surgery leads to early aseptic loosening, the most common cause of revision. The optimum method of cementation has not been fully evaluated. This study aimed to determine the effect of the acetabular component flange on mean and peak pressure during component insertion.

Method: A 53mm deepened hemisphere was machined from aluminium. Pressure transducers were positioned at the rim, at 45 degrees, and at the base. Polyethelene acetabular components of different sizes and flange designs were mounted onto a materials testing machine and inserted at a constant rate into Palacos R cement within the aluminium hemisphere. Insertion was stopped at a pre-determined point when an even cement mantle was achieved. The same components were then tested without a flange. Each test was repeated six times. Output data from the transducers was analysed.

Results: Components with a flange create a mean pressure 6–18 times higher at the rim than those without a flange. At the base pressures are 2–4 times higher. A stiffer flange generates higher peak and mean pressures than a more malleable flange. Delaying insertion by one minute does not increase the pressures achieved unless a flange is used.

Discussion: These results strongly support the use of a flange to contain cement during insertion of the acetabular component. Unflanged components fail to achieve satisfactory mean or peak pressures, even if insertion is delayed. This is likely to result in poor cement penetration into bone and reduced longevity of interface fixation.

With the development of new implants there is an increasing need for biomechanical studies. The problem of obtaining human specimen is well appreciated. Porcine spines are commonly used. To date there are no studies delineating the anatomy of porcine thoracolumbar vertebrae. The objective of this study is to provide a comprehensive database of measurements for the porcine thoracolumbar vertebrae with a view to help plan future studies contemplating their use.

6 adult porcine spines from 18–24 month old male pigs weighing 60 to 80 kilograms were obtained and dissected of soft tissue. The lowest thoracic and all the lumbar vertebrae were used in our experiment (n=42). 15 anatomical parameters from each vertebra were measured by 2 independent observers using digital calipers (Draper® PVC150D, accuracy ± 0.03mm). The mean, SD and SEM were calculated using Microsoft Excel. Results were compared with available data on human vertebra (Panjabi et al 1991,1992; Zindrick et al 1987; Kumar et al 2000).

The inter class correlation coefficient for the observers was 0.997. The intra-observer agreement was statistically robust (0.994). The vertebral bodies of the porcine vertebra were larger while both the upper and lower endplate depth and width were smaller than the human specimens. The pedicle width and depth was greater than the human specimens. The spinal canal length and depth of the porcine spine were smaller than humans indicating a narrow spinal canal. The spinous process length showed an increase from T16 to L1. This was in contrast to human spinous process.

This study provides a comprehensive database of anatomical measurements for the porcine thoracolumbar vertebra and highlights the differences in morphometry. These should borne in mind when designing studies using porcine spines and the implants matched accordingly. The measurements are also useful when extrapolating data from studies where porcine spines have been used.

Introduction: Aseptic loosening is the main cause of revision in hip replacement surgery. Improved cementation techniques have reduced the rate of loosening of the femoral component, leaving the cemented acetabular cup as the major problem, with reported loosening rates as high as 25% at 12 – 15 years. The ideal method of acetabular cementation has not been fully evaluated.

Aim: To determine the ideal thickness of cement mantle to resist torsional forces.

Method: Mahogany blocks with a 54mm hemispherical hole were used to simulate an acetabular socket. Machined aluminium cups were created in 5 sizes (52mm to 44mm) to give a cement mantle that varied in size from 1mm to 5mm. Three 10mm keyholes were drilled in the blocks and appropriate-sized spacers were inserted to ensure the mantle was accurate and even. Silicone grease was used to prevent any micro-interlock between cement and wood. The cups were then cemented into the wooden blocks using vacuum-mixed Palacos R cement and left to cure in air for 7 days at 37 °C. The constructs were tested to failure using a servo-hydraulic testing machine. Each experiment was repeated six times.

Results: The stiffness of the cement mantle varied according to thickness as follows:

Discussion: A stiffer cement mantle will transfer more torque to the bone-cement interface, possibly leading to earlier loosening of the prosthesis. This biomechanical analysis suggests that surgeons should aim to achieve a mantle at least 2mm thick. There appears to be little further mechanical advantage gained if the mantle is increased in thickness beyond 4mm.

Purpose: The study looked at different suturing configurations for hamstring reconstruction of the ACL to ensure that all four limbs of the graft were evenly tensioned at the time of insertion.

Method: Calf hoof flexor tendons were used as an animal model for the four strands of a “hamstring” ACL graft. They were sutured together at one end, which was clamped rigidly in a test rig. The free ends were then sutured altogether, in doubles or singly using 2 Ethibond. Miniature Variable Resistance Reluctance Transducers (DVRT) were then inserted into the tendons. The tendon construct was then loaded to 100N in 5N increments (with an initial I5N preload).

One hundred Newtons was decided on as the maximum force following measurements in theatre showing a force of approximately 75N were applied to a hamstring ACL graft at the time of fixation in the tibial tunnel. The DVRT’s allowed accurate assessment of the Load/Displacement of each tendon in the construct to be assessed. Repeatability experiments on the DVRT’s were performed. Ten specimens were prepared which were all tested in the different suturing configurations. Each test was repeated three times on each construct.

Results: The results showed that those tendons sutured all together did not load evenly with only one or two of the tendons in the construct showing any displacement throughout the loading cycle. When sutured as pairs, one of the tendons in each pair showed immediate displacement with the other tendon also showing displacement early in the loading cycle. When sutured individually the tendons all showed evidence of displacement early in the loading cycle, therefore showing more even tensioning and load sharing.

Conclusion: We recommend the use of singly sutured hamstring tendons in ACL reconstruction to allow more even tensioning and load sharing therefore reducing the risk of construct rupturing.