Hip resurfacing has many advantages such as proximal bone conservation and easy revision including conversion to total hip arthroplasty. The major complication in the hip resurfacing is notching at the lateral cortical bone and fracture of the neck. In this research, we simulated the range of direction of reaming without causing notch.

One left femur model was used for the simulation. The femoral head was fitted by a sphere and the origin of Cartesian coordinate was set at the center of the sphere. The simulation was made by imposing a cylindrical cut to the femoral head in varying direction and location. The existence of notching was decided comparing the maximum distance from reaming axis to neck section contour and the radius of cylindrical cut. If the maximum distance is bigger than the radius of cut, the notching exists and vice versa. We simulated existence of notching by varying inclination(α) from 20 to 70 degrees, anteversion(β) from 0 to 30 degree and depth passing through the head center(d) from 0 to 5mm. The implant used for the simulation was Durom^®, Zimmer^©. We selected the implant size that is close to the fitted sphere of femoral head.

No notching was made for any direction when the depth d was less than 2mm. When the depth was 3mm, notching did not generate in the range of α from 43 degrees to 60 degrees and β from 0 to 25 degrees. When the range of depth was from 4mm to 5mm, notching did not generate in ranges of α from 41 degrees to 60 degrees and β from 0 to 29 degrees. The no-notching angle range had tendency increasing slightly when the depth was increased. The angle between the stem of the implant and the neck shaft axis without notching can be calculated from the angle α. When the depth was from 4mm to 5mm, the corresponding angle between stem of implant and the shaft axis was from 120 degrees to 139 degrees.

Typical navigation system to insert hip implants in the accurate position consists of a 3D position measurement device and a computer. These navigation systems are classified into two categories according to the method of identifying the anterior pelvic plane that works as the reference of the orientation of the acetabulum cup. The preparation process for imageless navigation system is very easy because it uses three anatomical bony markers to define the anterior pelvic plane. When these anatomical bony markers are hard to locate, especially at the pubic symphysis due to the thick soft tissue, the accurate direction of the cup cannot be secured. The aim of this study is to estimate the soft tissue thickness without using the patient’s specific data such as the A-mode ultrasound image or C-arm image.

In our previous study, it was pointed out that the thickness of the hypodermic fat obtained through an ultrasound image could be estimated using the patient’s BMI and the displacement created by a specific force. Considering the probe shape, the soft-tissue thickness estimation formula is expressed as follows:

\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[Y_{estimated_thickness}\ =\ k(b_{0}\ +\ b_{1}\ {\times}\ BMI\ +\ b_{2}{\times}\ {\delta})\] \end{document}

k: constant for the shape of the probe end

Only two kinds of the probe end shapes (flat-ended probe and spherical-ended probe) were considered, and the change in the k value corresponding to the radius was calculated using the FE model of the soft tissue for each subject. The finite-element model was constructed as axisymmetric.

The simulation result of the initially assumed variables and the measured result were compared, and the optimization method was used to minimize the error: The RMS difference between the result of the experiment and that of the analysis was taken as the objective function. With the FE analysis for the two kinds of probe shapes with one subject, we determined the shape variable (k).

From the formula composed by a model with data from 28 people, the average error was 3 mm equivalent to the angle error of less than 1°. Therefore, the use of the method suggested in this study will help to improve the acetabulum cup navigation in THA, when we use only the surface points on the soft tissue. In addition, it seems that the soft-tissue thickness estimation formula suggested in this study may be generally used.

Purpose of the study: Dislocation of total hip arthroplasty remains a frequent complication, occurring for 0.5% to 10% of implants depending on the series. In about 30% of the cases, the orientation of the acetabular cup is involved. It is sometimes difficult to visualize the acetabular landmarks during surgical procedures performed for revision or with a minimally invasive technique. The surgeon uses the position of the pelvis on the operative table as a guide. It can be noted however that the patient’s weight bearing on the table is not constant during the entire operation, potentially changing the position of the pelvis during the procedure. We evaluated the use of a visual referential visible within the operative field for implanting the prosthetic cup.

Material and methods: The method materialized the anterior plane of the pelvis then transferred geometrically this plane for display on the ipsilateral iliac crest. The pelvis was masked under a drape, in lateral decubitus. The cup was implanted 12 times using the plane of the floor as the reference, 8 times using the acetabular rim as the reference, and 10 times using the iliac reference. The goal was 20° anteversion in the sagittal plane and 45° inclination in the frontal plane. The position of the pelvis was randomized. The final positions of the cup, of the iliac reference, of the anterior plane of the pelvis and of the floor were recorded with an optical system. Spearman’s test was used to search for correlations.

Results: Using the floor referential, mean anteversion was 21.8° (15–30.9°) and mean inclination 43.2° (37–47.6°). Using the acetabular referential, mean anteversion was 21.7° (18.1–26.6°) and mean inclination 45.8° (40.9–48.6°). With the iliac referential, mean anteversion was 20.3° (17.3–25.5°) and mean inclination 43.3° (41.1–44.8°). Mean error between the pelvic plane and the iliac referential was, for anteversion −0,32° (−1.07 to 0.8°) and for inclination, −0.1° (−0.95 to 1.43°). Implantation with the iliac referential was not correlated to the position of the pelvis. When the plane of the floor was used, the position of the implant was correlated with pelvic anteversion (p< 0.01) and inclination (p< 0.01).

Discussion: Insertion of the cup was independent of the position of the pelvis within a 3D referential in the operative field. In addition to computer-assisted navigation, simple tools can be developed to improve the surgeon’s perception in difficult indications, especially when they can provide satisfactory accuracy. A clinical feasibility study is currently under way.

Purpose: In-vitro mechanical tests are often used to pre-clinically assess the primary stability of hip endopros-theses. There is no standard protocol for these tests and the test conditions used vary greatly. This study examined the effect of the abductor muscle and the anterior-posterior component of the hip contact force (Fap) on the primary stability of cementless stems.

Methods: Cementless stems were implanted in 12 composite femurs which were divided into two groups: group 1 (N=6) was loaded with the hip contact force only, whereas group 2 (N=6) was additionally subjected to an abductor force. The cranial-caudal component of the hip contact force was the same in both groups, i.e. 2.3BW at 13° from the femur long axis. Each specimen was subjected to three Fap levels: 0, 0.3BW (walking), and 0.6BW (stair climbing). The implant translation relative to the femur was measured using a custom-built system comprised of 6 LVDT sensors. The resultant migration and micromotion were analyzed using an ANOVA with the abductor a between-group factor and Fap a within-group factor, followed by SNK post-hoc analysis with a significance level of 95%.

Results: Implant motion was not significantly affected when the Fap was increased from 0 to 0.3BW. However, without abductor, increasing Fap from 0.3 to 0.6BW increased migration and micromotion by an average of 291& #956;m (285% increase), and 15& #956;m (75%) respectively. With abductor, increasing Fap to 0.6BW increased migration by 87& #956;m (79%) but did not affect micromotion. The abductor did not significantly affect stem motion at lower Fap, but at Fap = 0.6BW motion was 50% lower compared with hip contact forces only.

Conclusions: Based on these results, inclusion of either abductor and/or Fap has little effect on implant motion when simulating walking. However, stair climbing (higher Fap) generates greater bone-implant motion compared to walking loads, and this effect is greatest in the absence of an abductor force. Funding: Other Education Grant Funding Parties: The Michael Smith Foundation for Health Research

In the cementless total hip arthroplasty, the position of the stem is pretty much determined by broach and rasping with which the is required for two reasons: one is to align the stem with the femur at the desired position and the orientation. The other is to achieve the conformity between the stem and the prepared proximal cavity surface in the femur. The robotic hip surgery can be a solution for the accurate of femoral canal shaping, but recent reports about the clinical follow-up study of the robotic hip surgery indicated frequent dislocation mainly due to the excessive soft tissue damage during robotic operation. In this paper, a guide being inserted into the femoral canal is proposed to restrict the undesired motion of the rasp inside the femur without extra incision.

A set of canal guide and custom rasp for the selected stem(versys fibermetal midcoat, zimmer co.)were developed and tested with 4 synthetic femurs (model 1130, Sawbones co.). After rasping, a plastic copy of the stem was inserted into the femur and sliced at 5 mm thickness. From obtained cross sections, percentages of the gap larger than 0.3mm between the stem and the bone was measured. 6_C_Results: In average, 79% of bone-implant interface was close contact. Valgus/varus deviations of the stem were 0.40±0.45 degree, which means the angle of axis of straight reamer and axis of final cut.

In average, 79% of bone-implant interface was close contact. Valgus/varus deviations of the stem were 0.40±0.45 degree, which means the angle of axis of straight reamer and axis of final cut.

The conformity of femoral canal with the femoral stem in this approach was higher than the conventional hip surgery and comparable to those in the robotic surgery. The alignment of the stem within the femur is also as good as those in the robotic surgery(0.34±0.67 approach does require neither expensive system nor CT scan. Also this approach can be executed swiftly without extra time and unnecessary large incision compared with the robotic surgery.

Heat generation during bone cutting operations inorthopaedics may cause thermal damage to the bone. During the bone cutting, the maximum temperature occurs on the contact surface between the bone and tool. Because of the low thermal conductivity and diffusivity, the temperature gradient of the bone interior is very high around the cutting site and the measurement of maximum temperature is difficult at the contact surface. While many researchers tried to measure the temperatures, they may have underestimated the temperatures of bone on account of measurement limitations. To solve this problem, we investigated the temperature distribution model of the bone interior during the milling operation and verified the model with a cutting experiment.

During the bone milling, most of the cutting energy is converted into the heat energy near the contact surface between the bone and tool. If the cutting tool moves on the bone surface, we can assume that a heat source moves on the bone surface at the speed of the feed rate. To predict the maximum temperature, we performed a milling experiment with fresh bovine cortical femurs. The feed rate were 2~9.8mm/s, the cutting depth were 0.3~1mm and the rotational speed were 30,000~50,000RPM. No irrigation solutions were applied. To measure the local temperatures around the tool, two infrared thermometers were attached behind the bur at 10mmintervals from the bur center. We calculated the maximum temperatures and errors from the measured temperatures.

The predicted maximum temperature increment was 55~131& #8451; as the cutting conditions change. The mean errors and standard deviation errors were several degrees. The increased feed rate and decreased cutting-depth reduced the maximum temperature.

Our observed temperature is quite higher than those in the previous studies. Because of the high temperature-gradient(57& #8451;/mm), the thermocouple alone will likely yield large errors and generally underestimate the temperatures of the bone interior. With a thermal damage criterion of 50& #8451;(& #916; T=13& #8451;), thermal damage may reach 1mm in depth. To reduce the thermal damage, it is recommended to increase feed rate and decrease cutting depth.

The purpose of this research is to propose CT-free cup orientator using tilt sensors without expensive point tracking devices in total hip replacement. In the case of using a mechanical guide, the accuracy of cup orientation can be sacrificed because of change of the patient’s posture during procedure. Several navigation systems have been introduced to secure an accurate position and orientation of the implant in THR. These systems are expensive and have some weakness due to possible interference inoptical measurement. Our orientator employs a T-bar shaped gauge and economic tilt sensors to secure a fairly orientation of acetabular cup inTHR.

The T-bar gauge having three feet with adjustable distance is designed to obtain the anatomical landmarks concurrently. Each foot is placed on the anatomical landamark of the sawbone. The gauge has its own tilt sensor to identifiy the tilt angle of the guage using AD input board. Similary, the cup positioning tool and dynamic reference base (DRB) have their own tilt sensors. The experimental procedures of CT-free cup orientator are done as follows:

Place the T-bar gauge in right place on the pelvis by setting three feet on the ASIS and pubic.

We define errors as difference between experimental data and ground truth obtained by Micro-Scribe (Immersion Inc.) Errors of the cup in abduction and anteversion were 1.2 and 1.0 degrees respectively when the test is performed on a sawbone.

We analyzed the causes of error to improve the accuracy of our cup orientator. Measuring landmarks and aligning three tilt sensors seemed to cause some errors. Base on this study, we expect to make an experiment on cadaver.

In clinical studies of cemented total hip arthroplasty (THA), polished stems produce less slippage at the bone-cement interface than roughened stems. Our objective is to assess the effect of stem-cement debonding on the bone-cement interface shear behaviour of hip implants using simplified axisymmetric stem-cement-aluminum models.

We emulated the femoral stems using stainless steel tapered plugs with either a rough (i.e. bonded) or smooth (i.e. unbonded) surface finish. Three different taper angles (5°, 7.5°, 10°) were used for the unbonded constructs. Non-tapered and tapered (7.5°) aluminum shells were used to emulate the diaphyseal and metaphyseal segments of the femur. In all cases, the cement-aluminum interface was designed to have the same shear strength as has been reported for bone-cement interfaces (~8 MPa). The test involved applying axial compressive loading at a rate of 0.02 mm/s until failure. Six specimens were tested for each combination of the parameters.

The unbonded stems sustained about twice as much load as the bonded stem, regardless of taper angle, and the metaphyseal model carried 35-50% greater loads than the diaphyseal models before shear failure or slippage. The unbonded constructs reached peak load with excessive displacement due to creep of the cement mantle while the bonded constructs failed in shear at the cement-aluminum interface. This result supports the hypothesis that the wedging forces created in the unbonded construct increase the compression forces across the aluminum-cement interface, thereby increasing its shear resistance. A finite element analysis predicted that the cement could withstand the hoop stress under these loading circumstances and this prediction was confirmed by visual inspection of the cement after each test.

Our results suggest that smooth or unbonded stems should sustain less slippage and shear damage at the bone-cement interface than roughened or bonded stems due to the wedge-induced compressive stress; this increased load capacity will be particularly valuable when the condition of the bone-cement interface is suboptimal.