In Vitro Estimation of Initial Stability of a Cementless Stem Made of Shape Memory Alloy

International Journal of Automation and Power Engineering (IJAPE) Volume 4, 2015 www.ijape.org doi: 10.14355/ijape.2015.04.001

In Vitro Estimation of Initial Stability of a Cementless Stem Made of Shape Memory Alloy Masaru Higa*1, Takuya Tsuchihashi1, Tomoki Jonoshita1, Masayoshi Abo1, Satoshi Kakunai1 Department of Mechanical Engineering, University of Hyogo, Himeji, Japan

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*1

higa@eng.u‐hyogo.ac.jp

Abstract The success of total hip arthroplasty (THA) in improving joint function and reducing pain is regarded as one of the great achievements of modern medicine. For a cementless THA stem, however, lack of initial stability leads to thigh pain and eventual loosening of the stem because of a loss of bone integration on the stem surface or continuous bone resorption around the stem. Obtaining adequate initial stability is one of the important factors for good short‐term and long‐term clinical outcomes.

While most of the cementless stems are made of either cobalt‐chrome‐molybdenum (CoCrMb) or titanium‐aluminum‐ vanadium (TiAlV) alloys, we have focused on an expandable cementless stem utilizing nickel‐titanium (Ni‐Ti) shape memory alloy (SMA). In this study, the expandable stem with SMA was evaluated in vitro, specifically testing to determine if the stem is able to achieve rigid initial stability in a medullary cavity under cyclic loadings. The proposed stem has Ni‐Ti SMA expandable blades on the stem surfaces. For shape memorization, the SMA was restrained at an expanded shape under appropriate thermo‐ mechanical treatments. At the time of implantation into bone, the SMA with a contracted shape was kept below the martensite transformation temperature. After implantation, the SMA blades undergo reverse transformation due to body temperature, accompanied by a change from the contracted shape to the expanded shapes. The shape change results in a residual pressure between the bone and the stem. In mechanical tests, the stem with SMA showed better initial stability, in terms of relative micromotions at the stem‐bone interface, under cyclic loadings. After 120 loadings, micromotion of 11 m was observed on the stem with SMA, while the micromotion on the stem without SMA was 122 m. Use of an expandable stem utilizing SMA is effective enough for rigid initial stability.

Keywords Total Hip Arthroplasty (THA), Shape Memory Alloy (SMA), Cementless Stem, Implant Fixation, Micromotion, In Vitro Testing

Introduction Total hip arthroplasty (THA) has been used in both primary and revision surgery. In spite of good mid‐ and long‐ term clinical results, a great majority of cementless stems have been revised (Malchau et al. 2005). A cementless femoral component (cementless stem) must be placed in contact with the bone with sufficient strength to support the stem rigidly, which otherwise would require a revision surgery (Berry 2003). Aseptic loosening, in which a stem loosens from the host bone, is one of the major reasons for revision surgery. The clinical rationale of cementless stem fixation is to achieve rigid initial stability through press‐fit anchorage and secondary stability through bone integration on the stem surface (Heller et al. 2005). Hence, rigid initial stability of an intramedullary stem is essential to create long‐term stable fixation and to create functional load transfer, and may result in minimal bone loss. A large number of clinical and experimental studies of cementless THA have shown the importance of rigid initial stability for the mid‐ and long‐term clinical complacence of the cementless stems (Burke et al. 1991, Pancanti, Bernakiewicz, and Viceconti 2003, Berry 2003).

Various cementless stem designs and materials have been developed to achieve rigid initial stability and therefore to ensure sufficient long‐term stability. Recently, a unique approach that uses an “expandable” stem was reported to provide rigid initial stability of the stem. The expandable stem utilizes bone cement to expand a stem cross section that controls pressure after the stem is implanted. The theoretical advantages of this design include the ability of the expandable stem to conform to the natural variability in the internal contours of the proximal femoral shaft, improved fixation, and the simplicity of using one component size for all patients (Kummer, Strauss, and Jaffe 2007). With this technique, however, careful attention must be paid to controlling cement viscosity. Moreover,

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the stem is filled with bone cement instead of a metal, so the strength of the stem may be insufficient. Similar to the concept of expandable prostheses, a proximal femoral nail for peritrochanteric fractures (Folman et al. 2006) and a hemi hip replacement stem for femoral neck fractures (Folman et al. 2010) have also been developed. Their expandable system, which uses pressurized saline solution to expand the nail or the stem, consists of membranes and four longitudinal bars with a hollow structure. Although their stem showed good fit and fill to the host bone intramedullary, the fatigue strength of this stem is doubtful. An extra designated pump and a pressure gauge are also required. An expandable knee reconstruction for skeletally immature patients (Eckardt et al. 2000) was also reported. These examples clearly indicate the need for a prosthesis that has enough strength for simple operative procedures as well as rigid initial stability.

Considering the recent developments and problems with stem designs and materials, we proposed using a cementless stem utilizing shape memory alloy (SMA) to achieve rigid initial stability (Higa et al. 2010). This cementless stem, which is also expandable intramedullary, can be implanted using standard surgical procedures without any special precautions or extra equipment, and has sufficient mechanical strength. SMA exhibits two unique characteristics when subjected to appropriate thermo‐mechanical procedures. One is the ability to return to an original shape, called the shape memory effect (SME). The other is pseudo elasticity (PE). Medical applications with SMA utilizing SME and PE are extremely successful thanks to SMA’s chemical and mechanical stability inside the body, as well as its biocompatibility (Machado and Savi 2003, Assad et al. 1998). We developed the cementless stem with SMA utilizing SME with accompanying shape change. While some patents claim the use of SMA as a material for THA (Nakanishi and Shibata 2004), the focus of these patents is on the use of SMA to provide sufficient fatigue strength and outstanding biocompatibility. Thus, our idea will be compatible with the claim made by the manufactures owing to the fact that the claim never encompasses the shape change.

Estimating the magnitude of the stem movements relative to the bone for many different types of stem anchorage may help to evaluate the usefulness of a new technique to rigidly secure a cementless stem. Thus, we had proposed the theoretical possibility of the stem with SMA using the finite element method (Higa et al. 2010). However, our theoretical study lacked experimental validations. Therefore, this study aimed to verify the initial stability of the cementless stem with SMA in vitro. To accomplish this, the relative micromotions at the bone‐stem interface were measured under cyclic loadings in vitro. The hypothesis tested in this study is that a stem with SMA would exhibit a smaller degree of micromotion than a stem without SMA. Materials and Basic Structure Nickel‐Titanium (Ni‐Ti) SMA (Ti 51 at. % Ni) was adopted in this study (MEMOALLOY, NEC TOKIN, Sendai, Japan). This material has been used in various fields as a part of medical devices, such as orthodontic appliances, artificial hearts, bone plates and catheters. The proposed THA stem consists of two expandable SMA blades (3.5 mm in width and 0.6 mm in thickness) jointed at both the anterior and posterior surfaces of the stem. In this study, two types of SMA blades were tested (Figs. 1 and 2). The stem was simplified as a tapered cylinder. For shape memorization, the SMA blades were subjected to appropriate thermo‐mechanical procedures, such as a solution treatment (850°C, 20 min) followed by an aging treatment (400°C, 1h) with restrained the two types of expandable shapes. After the termo‐mechanical procedures, the phase transformation temperatures were obtained by measuring the electrical resistivity by the four‐probe method while changing material temperatures. The material temperatures were monitored by a T‐type thermo couple and were changed in an incubator from room temperature (19°C) down to −100°C, up to 70°C, and then back to room temperature. This cycle was repeated twice for each material. The phase transformation temperatures were acquired from electrical resistivity versus temperature curves: start of martensite transformation (Ms) is −5°C, finish of martensite transformation (Mf) is −55°C, start of reverse transformation (As) is 20°C, and finish of reverse transformation (Af) is 30°C, respectively (Fig. 3). These transformation temperatures can be set by adjusting the aging treatment conditions. The complete shape recovery of the SMA requires a temperature increase from a temperature below Mf to a temperature slightly above Af. Hence, the stem with SMA blades must be implanted into the femur keeping a temperature below 20°C (T<As). Upon heating, reverse transformation occurs in the SMA blades accompanying the change from the contracted shape to the expanded shape. In the medullary cavity, however, the supposed shape changes do not occur because the stem is surrounded by cortical bone. This situation produces high contact pressure at the bone‐ stem interface.

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FIG. 1 THESE TWO PICTURES SHOW THE THA STEMS WITH SMA AT THE EXPANDED SHAPES OF TWO TYPES: TYPE 1 SMA (LEFT) AND TYPE 2 SMA (RIGHT).

FIG. 2 CHANGES IN SHAPE OF TWO TYPES OF SMA BLADES; TYPE 1 (LEFT) AND TYPE 2 (RIGHT) DURING THE PHASE TRANSFORMATION. UPON HEATING, THE REVERSE TRANSFORMATION OCCURS FROM MARTENSITE PHASE (BOTTOM) TO AUSTENITE (TOP) ACCOMPANYING SHAPE CHANGES FROM FLAT (CONTRACTED) TO THE EXPANDED SHAPE.

FIG. 3 ELECTRICAL RESISTANCES OF THE SMA PLOTTED AS A FUNCTION OF TEMPERATURES DURING BOTH COOLING AND HEATING PROCESSES. THE PHASE TRANSFORMATIONS CAN BE DETECTED BY RAPID CHANGES IN THE ELECTRICAL RESISTANCE. HENCE, ABSOLUTE VALUES OF THE VERTICAL AXIS ARE OMITTED. START AND FINISH TEMPERATURES OF THE PHASE TRANSFORMATIONS ARE AS FOLLOWS: START OF MARTENSITE TRANSFORMATION (Ms) IS −5°C, FINISH OF MARTENSITE TRANSFORMATION (Mf) IS −55°C, START OF REVERSE TRANSFORMATION (As) IS 20°C, AND FINISH OF REVERSE TRANSFORMATION (Af) IS 30°C.

Experimental Method Specimen Preparation and Experimental Setup The THA models used in this study were idealized as an axisymmetric geometry consisting of a circular hollow femur replica of homogeneous isotropic cortical bone (3403‐6, Pacific Research Laboratories, Inc., Vashon, WA, USA) and a tapered stainless steel (SUS 304) cylinder replicating a proximal part of a cementless stem with or

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without SMA blades. The stem is 28 mm in diameter with a 1:10 taper. The proximal edge of the femora was slightly cut by a lathe to anchor the stem at an appropriate position.

Three stem configurations were prepared: with Type 1 SMA, with Type 2 SMA and without SMA. Before insertion, the stems were cooled by liquid nitrogen (T<Mf) with a contracted shape, i.e., the blades were not extended. There is less resistance when inserting the stem with this contracted shape. Then, each stem was press‐fitted into the femur by applying a 300 N axial compressive load. During the insertion, the stem temperature was kept under 20°C (T<As) to avoid starting the reverse transformation. The stem was heated by air after the insertion (T>Af). The temperature of the stem was monitored by a T‐type thermocouple during the whole experiment. For measurements of the micromotion at the bone‐stem interface, a trans‐cortical hole was drilled on the distal femur at corresponding locations to allow measurement of the micromotion. At a location on the outside surface of the femur, a high‐precision linear variable displacement transducer (CE‐5, Tokyo Sokki kenkyujo Co., Ltd., Tokyo, Japan) was attached with a custom‐made jig. Both outputs from the thermocouple and the displacement transducer were recorded through the hole at a frequency of 100 Hz by a digital data logger (NR‐500, KEYENCE Co., Osaka, Japan).

Mechanical Tests of the Stability The THA models were loaded using a uniaxial mechanical testing machine (4204, INSTRON, Norwood, MA, USA).

The femur was fixed perpendicularly at the distal end completely by cement. Axial loads of 300 N were applied to the top of the stem. This relatively small load was chosen to limit potential fracturing of the femur during repeated loading. All specimens were loaded 120 times in 0.1 mm/min increments up to 300 N and then unloaded to 0 N. The total interface micromotions were the summation of the elastic (recoverable) and the plastic (unrecoverable) movement components (Otani et al. 1993). Elastic movements were averaged for each stem configuration at the 1st and 120th loadings. Subsidence was defined as the amount of cumulative plastic movement. During experiments, room temperature was set to avoid starting martensite transformation (T>Ms) and was monitored continually by the thermocouple. Four specimens were tested for each configuration, thus 12 (4×3) specimens were prepared in total. All results were statistically compared between the three groups using an analysis of variance technique.

Experimental Results Cumulated subsidences for the three configurations are plotted in Fig.4. The largest subsidence registered after the 120th loading was 122.4 m without the SMA plate, while the subsidence for type 1 SMA was 25.2 m. The smallest subsidence of 11.0 m was observed for type 2 SMA. After the 120th loading, cumulative subsidence with type 1 SMA was significantly smaller compared to the subsidence without SMA blades (p<0.018). The subsidence of type 2 SMA was also significantly smaller than without SMA (p<0.006). Subsidences of the two types of SMA blades are not significantly different (p=0.11) at the 120th loading. In all three configurations, the subsidences increased markedly at the early phase of the loading then gradually saturated. These increasing rates per loading are plotted in Fig.5. The increasing rates of the subsidence were saturated at the 100th loading for the stem without SMA, at the 31st loading for type 1 SMA, and at the 21st loading for type 2 SMA (Fig.5).

FIG. 4 CUMULATIVE SUBSIDENCES OF THE THREE STEM CONFIGURATIONS. MEAN AND STANDARD DEVIATIONS ARE PRESENTED.

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FIG.5 RELATIVE SUBSIDENCES OF THE THREE STEM CONFIGURATIONS AGAINST LOAD CYCLES ARE SHOWN. THESE CURVES ARE DERIVATIVES OF THE CURVES IN FIG. 4.

ALL THREE CURVES CONVERGED TO ZERO AT DIFFERENT RATES

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Mean values of elastic micromotion at the 1st and 120th loadings were 18.8 m and 15.5 m for without SMA, 16.3 m and 17.7 m for type 1 SMA, and 12.7 m and 12.7 m for type 2 SMA, respectively (Table1). Although the elastic micromotion of type 2 SMA showed the smallest value, they were not significantly different between the three groups at any time.

TABLE 1 MEAN (SD) SUBSIDENCE AND ELASTIC MICROMOTIONS OF THE THREE CONFIGURATIONS: WITHOUT SMA, TYPE 1 SMA AND TYPE 2 SMA. SUBSIDENCES OF BOTH TYPES OF SMA ARE SIGNIFICANTLY SMALLER THAN OF THE CONFIGURATION WITHOUT SMA. ELASTIC MICROMOTIONS ARE NOT SIGNIFICANTLY DIFFERENT.

Without SMA

Type 1 SMA

Type 2 SMA

122.4 (33.7)

25.2 (18.4)

11.0 (5.1)

at the 1st loading

18.8 (3.7)

16.3 (3.9)

12.7 (4.7)

at the 120th loading

15.5 (6.0)

17.7 (3.4)

12.7 (6.2)

Subsidences at the 120th loading Elastic micromotions Unit: m

Discussion Expected Positive Effects of the Proposed Stem In the present study, initial fixation of a newly designed cementless stem with SMA blades was assessed in vitro.

The stem with SMA showed smaller micromotions under cyclic loadings. This indicates that the stem has rigid initial stability. Every stem, on the other hand, must be retrievable in anticipation of a revision surgery. Although rigid fixation is desirable, the stem should not always be anchored to the host bone. For this reason, the two types of SMA blades were swept downwards. These SMA “teeth”, which prohibited inferior micromotions under loading, would not be an obstacle to revision surgery. This concept is more advantageous than other existing expandable prostheses. The SMA blades exhibited almost no motion. Thus, the incomplete shape change produces a compression force at the bone‐stem interface. This compression force effect is similar to what can be achieved by means of the press‐fit implant technique. Rigid initial stability is believed to rely on achieving a sufficient interface press‐fit between the stem and bone. However improper sizing of the canal and inadequate insertion of the stem can lead to femoral canal fracture when attempting to establish an interface press‐fit. The SMA shape change produces the press‐fit effect automatically after insertion. It is firmly recommended that this compression force should be controlled by means of a thermo‐mechanical treatment.

Force Measurements of the SMA Blades In the medullary cavity, the phase transformation on the SMA does not result in complete shape change but in high contact pressure at the bone‐stem interface. There are few, if any shape changes. The concepts discussed above were evaluated by direct measurements of the expanding forces from the SMA blades. Two types of SMA blades were evaluated. Under a cooled condition (T<Mf), one end of a SMA blade (attached to the stem side) was fixed, and a one‐dimensional load cell (LMA‐A‐50N, Kyowa Electronic Instruments Co., Ltd., Tokyo, JP) was attached to the other side of the SMA blade. The plate was then heated until body temperature (37°C) while measuring the load cell outputs. Under this condition, however, the blade moved very little, because of restriction by the load cell.

Four blades were prepared for each type and measured three times for each specimen. Mean expandable forces were 8 N in type 1 SMA and 30 N in type 2 SMA. These small values, which are enough to reduce the micromotions, do not result in femoral fracture.

Current Issues with the Proposed Stem A major issue with the device is that there is a possibility of thigh pain resulting from the compression force at the bone‐stem interface. The incidence of thigh pain is higher in patients with cementless stems, although no evidence of thigh pain was reported in a 5‐year follow‐up of patients with the VerSys‐FMT femoral stem (Akhavan and Goldberg 2007). The cause of thigh pain has not been completely clarified. However, excessive endosteal stress or the state of the metal stuck on the endosteal surface as in type 1 SMA would likely be harmful. Although the compression force was 30 N at most, we cannot say if this compression force contributes to thigh pain. Moreover, stress concentration results in cortical hypertrophy or medullary sclerosis sometimes observed near the tip of stems.

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Our main objective in developing the stem with SMA is to enhance the initial stability. However, complications such as thigh pain must be kept in mind. Another concern with this device is how to manufacture it. Notably, the Ni‐Ti alloy is a very unworkable material. Although we manufactured two types of SMA blades using a rolling mill and a cutter, the blades and the stem were connected using screw connections, which are not sufficient for long‐term use.

Limitations of This Study This study has some limitations. One is that the stem was fixed to the cortical bone exclusively at the proximal and mid portion of the stem surface. Distal fixation was not possible because of the taper and polished surface of the stem. With this in mind, two expandable SMA blades were placed only at the proximal surface of the stem. There are many types of cementless stems, including tapered, cylindrical, and anatomic stems (Mai et al. 2010).

Cylindrical stems, such as long and extensively porous coated anatomic medullary locking type stems (AML, Depuy, Warsaw, IN, USA) and tapered stems, such as Zweymuller (Zimmer, Ltd., South Marston, UK) stems, are supposed to fit to the bone extensively. There is also another fixation concept, called three‐point fixation (Mulliken et al. 1996). Apart from these fixation concepts, however, we limit the discussion to the stem with a proximal fit, such as a tapered stem with proximal fixation, like the VerSys‐FMT stem (Akhavan and Goldberg 2007). The second limitation is that we used simplified stem and bone shapes. Although a more realistic model of the bone and stem is preferable, our specialized model can be used to assess the possibility of reducing the micromotion with SMA blades. We could also determine the superior blade type by comparing different types of blades.

Another limitation is that we measured the relative micromotion between bone‐stem interface until the 120th loading. Loading 120 times assumes just a short‐term postoperative condition. Therefore, the data are only applicable to the immediate postoperative period.

Conclusion This study showed that an expandable stem utilizing SMA is effective for obtaining rigid initial stability. We proposed two types of SMA blades. Although type 2 SMA showed smaller subsidence and elastic micromotion, no significant differences in elastic micromotion and subsidence were found between two types of SMA blades.

Contrary to this finding, we recommend the type 2 SMA rather than type 1, because type 2 has a smaller standard deviation in subsidence values, which means more stable fixation was achieved. Moreover, type 2 has a larger contact area, whereas type 1 blades only contact the endosteal surface. Although some concerns remain, rigid initial fixation of a cementless stem with SMA was achieved in vitro.

ACKNOWLEDGMENT

This research was partly supported by the grant‐in‐aid for young scientist (B) (20700382).

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