| Summary: | Primary stability of stems in cementless total hip replacements is recognized to play a critical
role for long-term survival and thus for the success of the overall surgical procedure. In
Literature, several studies addressed this important issue. Different approaches have been
explored aiming to evaluate the extent of stability achieved during surgery. Some of these are
in-vitro protocols while other tools are coinceived for the post-operative assessment of
prosthesis migration relative to the host bone. In vitro protocols reported in the literature are
not exportable to the operating room. Anyway most of them show a good overall accuracy. The
RSA, EBRA and the radiographic analysis are currently used to check the healing process of
the implanted femur at different follow-ups, evaluating implant migration, occurance of bone
resorption or osteolysis at the interface. These methods are important for follow up and clinical
study but do not assist the surgeon during implantation.
At the time I started my Ph.D Study in Bioengineering, only one study had been undertaken to
measure stability intra-operatively. No follow-up was presented to describe further results
obtained with that device.
In this scenario, it was believed that an instrument that could measure intra-operatively the
stability achieved by an implanted stem would consistently improve the rate of success. This
instrument should be accurate and should give to the surgeon during implantation a quick
answer concerning the stability of the implanted stem. With this aim, an intra-operative device
was designed, developed and validated. The device is meant to help the surgeon to decide how
much to press-fit the implant. It is essentially made of a torsional load cell, able to measure the
extent of torque applied by the surgeon to test primary stability, an angular sensor that measure
the relative angular displacement between stem and femur, a rigid connector that enable
connecting the device to the stem, and all the electronics for signals conditioning. The device
was successfully validated in-vitro, showing a good overall accuracy in discriminating stable
from unstable implants. Repeatability tests showed that the device was reliable. A calibration
procedure was then performed in order to convert the angular readout into a linear
displacement measurement, which is an information clinically relevant and simple to read in
real-time by the surgeon.
The second study reported in my thesis, concerns the evaluation of the possibility to have
predictive information regarding the primary stability of a cementless stem, by measuring the
micromotion of the last rasp used by the surgeon to prepare the femoral canal. This information
would be really useful to the surgeon, who could check prior to the implantation process if the
planned stem size can achieve a sufficient degree of primary stability, under optimal press
fitting conditions. An intra-operative tool was developed to this aim. It was derived from a
previously validated device, which was adapted for the specific purpose. The device is able to
measure the relative micromotion between the femur and the rasp, when a torsional load is
applied. An in-vitro protocol was developed and validated on both composite and cadaveric
specimens. High correlation was observed between one of the parameters extracted form the
acquisitions made on the rasp and the stability of the corresponding stem, when optimally
press-fitted by the surgeon. After tuning in-vitro the protocol as in a closed loop, verification
was made on two hip patients, confirming the results obtained in-vitro and highlighting the
independence of the rasp indicator from the bone quality, anatomy and preserving conditions
of the tested specimens, and from the sharpening of the rasp blades.
The third study is related to an approach that have been recently explored in the orthopaedic
community, but that was already in use in other scientific fields. It is based on the vibration
analysis technique. This method has been successfully used to investigate the mechanical
properties of the bone and its application to evaluate the extent of fixation of dental implants
has been explored, even if its validity in this field is still under discussion. Several studies have
been published recently on the stability assessment of hip implants by vibration analysis.
The aim of the reported study was to develop and validate a prototype device based on the
vibration analysis technique to measure intra-operatively the extent of implant stability. The
expected advantages of a vibration-based device are easier clinical use, smaller dimensions and
minor overall cost with respect to other devices based on direct micromotion measurement.
The prototype developed consists of a piezoelectric exciter connected to the stem and an
accelerometer attached to the femur. Preliminary tests were performed on four composite
femurs implanted with a conventional stem. The results showed that the input signal was
repeatable and the output could be recorded accurately.
The fourth study concerns the application of the device based on the vibration analysis
technique to several cases, considering both composite and cadaveric specimens. Different
degrees of bone quality were tested, as well as different femur anatomies and several levels of
press-fitting were considered. The aim of the study was to verify if it is possible to discriminate
between stable and quasi-stable implants, because this is the most challenging detection for the
surgeon in the operation room. Moreover, it was possible to validate the measurement protocol
by comparing the results of the acquisitions made with the vibration-based tool to two
reference measurements made by means of a validated technique, and a validated device. The
results highlighted that the most sensitive parameter to stability is the shift in resonance
frequency of the stem-bone system, showing high correlation with residual micromotion on all
the tested specimens. Thus, it seems possible to discriminate between many levels of stability,
from the grossly loosened implant, through the quasi-stable implants, to the definitely stable
one.
Finally, an additional study was performed on a different type of hip prosthesis, which has
recently gained great interest thus becoming fairly popular in some countries in the last few
years: the hip resurfacing prosthesis.
The study was motivated by the following rationale: although bone-prosthesis micromotion is
known to influence the stability of total hip replacement, its effect on the outcome of
resurfacing implants has not been investigated in-vitro yet, but only clinically. Thus the work
was aimed at verifying if it was possible to apply to the resurfacing prosthesis one of the intraoperative
devices just validated for the measurement of the micromotion in the resurfacing
implants. To do that, a preliminary study was performed in order to evaluate the extent of
migration and the typical elastic movement for an epiphyseal prosthesis. An in-vitro procedure
was developed to measure micromotions of resurfacing implants. This included a set of in-vitro
loading scenarios that covers the range of directions covered by hip resultant forces in the most
typical motor-tasks. The applicability of the protocol was assessed on two different commercial
designs and on different head sizes. The repeatability and reproducibility were excellent
(comparable to the best previously published protocols for standard cemented hip stems).
Results showed that the procedure is accurate enough to detect micromotions of the order of
few microns. The protocol proposed was thus completely validated. The results of the study
demonstrated that the application of an intra-operative device to the resurfacing implants is not
necessary, as the typical micromovement associated to this type of prosthesis could be
considered negligible and thus not critical for the stabilization process.
Concluding, four intra-operative tools have been developed and fully validated during these
three years of research activity. The use in the clinical setting was tested for one of the devices,
which could be used right now by the surgeon to evaluate the degree of stability achieved
through the press-fitting procedure. The tool adapted to be used on the rasp was a good
predictor of the stability of the stem. Thus it could be useful for the surgeon while checking if
the pre-operative planning was correct. The device based on the vibration technique showed
great accuracy, small dimensions, and thus has a great potential to become an instrument
appreciated by the surgeon. It still need a clinical evaluation, and must be industrialized as
well. The in-vitro tool worked very well, and can be applied for assessing resurfacing implants
pre-clinically.
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