EXPERIMENTAL IN-VITRO BONE CEMENTS DISINTEGRATION WITH ULTRASONIC PULSATING WATER JET FOR REVISION ARTHROPLASTY

Original scientific paper The paper deals with the study of using the selective property of ultrasonic pulsating water jet for the disintegration of the interface created by bone cement between cemented femoral stem and trabecular bone tissue as a potential technique for revision arthroplasty. Six types of commercial bone cements based on Polymethyl Methacrylate were used for investigation. The cements were mixed using the DePuy SmartMix® CTS / vacuum mixing bowl. Mechanical properties of hardened bone cements were determined by nanoindentation. The bone cement samples were disintegrated using the pulsating water jet technology. The water pressure varied between 8÷20 MPa. A circular nozzle with an orifice diameter of 0,7 mm was used for water jetting. The stand-off distance from the target material was 2 mm and the traverse speed 1 mm/s. The volume of material removal and depth of created traces were measured by MicroProf FRT optical profilometer. The results positively support an assumption that pulsating water jet has a potential to be a suitable technique for the quick and safe disintegration of bone cement during revision arthroplasty.


Introduction
Removal of bone cement during revision arthroplasty is a very important factor of this surgical technique.More than 20 % of all arthroplasties will need to be revised due to aseptic loosening, recurrent dislocation, infection or periprosthestic fracture.Revision arthroplasty has less favourable outcomes than primary.The removal of bone cement from the femoral canal extends over certain amount of time, which heightens the risk of complications, i.e. the risk of bone damage and even the danger of unsuccessful reimplantation of a new prosthesis.The extraction efficiency of cemented femoral stem depends on the technique, applied technologies, the material and nursing care (Fig. 1).Each prosthesis consists of femoral and acetabular components.The femoral component is inserted in the femur and bone cement forms the interface between the bone and bone cement.During hip replacement surgery, the bone cement must be disintegrated and then removed the femoral component is removed.Cemented endoprosthesis is inserted in the femoral channel.Mechanical properties of the femoral stem are determined by the type of material (titanium, stainless steel).
Many techniques for creating femoral windows have been described by Witt et al. [17], Hackenbroch et al. [18], Weber et al. [19], Tyer et al. [20], Moreland et al. [21].To obtain access to entire distal cement, distal fenestration and expansion of trochanteric osteotomy or the transfemoral approach, the authors Younger et al. [22], Chen et al. [23], Miner et al. [24] investigated the use of extended osteotomy.To reduce the cortical window and extended trochanteric osteotomy disadvantages, the authors Goto et al. [26] studied cement-in-cement techniques.This technique is characterized by removing part of old bone cement, with a new prosthesis then being inserted.New bone cement is added to the old, and the stem is fixed in the femoral canal.Cement-in-cement technique has been introduced to reduce intraoperative complications during and after the revision of fixed cemented stem.Some good preliminary results have been reported, but the perforation and fracture of the femur were recorded.
The next investigation area was the applications of lasers.The authors Buchelt et al. examined the application of laser for the disintegration of PMMA.Braren et al. [27] demonstrated the use of excimer laser for the material removal of bone cement, but in experiments this tool was very slow.Scholz et al. [28] found that carbon dioxide laser showed the best effect, but is currently used rarely.Caillouette et al. [29], deSteiger et al. [31] and Smith et al. [32] described a new ultrasonically-driven tool for the removal of bone cement.The author hit at the need for a cooling and sucking system.The first opportunities for PMMA bone cement removal by water jet were described by Honl et al. [33].Prof. Honl dealt with the use of water jet as a new tool for endoprosthesis revision surgery.Opportunities for the use of both plain and abrasive water jet as a cutting tool were investigated in study [34,35,36].

Experimental arrangement
The removal of bone cement during revision arthroplasty [37] is a very important aspect of the surgical technique.Bone cement removal has been studied by many authors and many techniques were developed in order to enhance bone cement removal during endoprostheses replacement.
The long-term success of the reimplanted endoprosthesis lies in the applied surgical techniques as well as the material which creates an interface between the femoral stem and trabecular bone tissue.During revision surgery, taking in account other factors (Fig. 1), many problems and complications occur such as cracks of bone and loosing biological potential.Mentioned issues are caused by the mechanical and thermal damage of bone tissue during classical technique surgery.Above described methods for the removal of bone cement include high-speed drills, chisels, saws, and reamers, which are often associated with the fracture or perforation of the femoral shaft.The removal of bone cement from the femoral canal takes a certain amount of time, and carries the risk of complications, thus risking bone damage and perhaps even endangering the successful reimplantation of a new prosthesis.Therefore the aim of the study is the experimental testing of a technique with low technological level set up, in order to simplify the introduction of a non-destructive, cold technique to the orthopedic field.In our experiment commercial PMMA bone cement samples were disintegrated using the pulsating water jet technology.The effect of water pressure on the volume of material removal and depth of created traces was investigated in particular.

Experiment
In order to investigate the possible use of water jet technology and the modifications thereof, bone cement samples were disintegrated using pulsating water jet.The tests were performed at the Institute of Geonics of the CAS in Ostrava.The technological set up consisted of a plunger pump Hammelmann HDP 253, and robot ABB IRB 6640-180 for handling the pulsating water head (Figs. 2 and 3).As a source of acoustic waves for the generation of pulsating water jet, an Ecoson WJ-UG_630-40 generator specially designed for pulsating water jetting tools was used.The MVT circular nozzle with an orifice diameter of 0,7 mm was used for water jetting.The standoff distance from the target material was 2 mm.The traverse speed was 1 mm/s.The pump pressure varied between 8 ÷ 20 MPa.The technological conditions of the tests are summarized in Tab. 1. Experiments were performed with pulsating water jet with the modulation frequency of 41,9 kHz.6 types of PMMA bone cement were investigated.The cement samples were prepared by manual mixing under vacuum in a DePuy vacuum bowl in strict compliance with the manufacturer's instructions.Microscopic preparations (polished sections) were prepared from the bone cements for assessment of their physical properties.The sections were polished down to a roughness of 0,1 μm.Size and volume of bubble pores in hardened bone cements were measured using image analysis methods.Nanoindentation measurements were performed on the polished sections with a TTX-NHT nanoindentation tester (CSM Instruments, Switzerland).A Berkovich pyramid diamond tip was used in simple monotonic mode with maximum loading of 10 mN.Loading and unloading rate was 20 mN/min, the hold time at maximum was 10 s.The resulting load-penetration depth (P-h) curves were evaluated according to the analysis of Oliver and Pharr [38].The results of mechanical properties estimation on hardened bone cement samples by the nanoindentation are shown in Tab. 2. The volume of material removal from the surface area of 4 × 4 mm was measured and the depth of created traces was determined using the MicroProf FRT profilometer.The mean depth of a cutting trace was calculated as an arithmetical average of 1000 values measured along the trace.

Results and discussion
The results of the bone cement disintegration tests are graphically illustrated in Figs. 4, 5, 6 and 7. Values of the material removal volume rise significantly with increasing water pressure (Fig. 4).An increase in water pressure from 8 to 20 MPa can cause more than three times higher values of material removal (see the Palacos sample in Fig. 4).There are no strong differences between particular cement types.It corresponds to the mechanical properties of the bone cements, which have similar values for all tested samples (Tab.2).Dependence of the mean depth of cutting trace   on the water pressure shows a similar increasing trend (Fig. 5).The depth of created grooves ranges between 0,5 and 2,8 mm and the differences between the tested bone cement samples are not significant.The character of traces created by the pulsating water jet is illustrated in Figs. 6 and 7.The traces have ragged bottom and sides.The cross-section profile shape mostly evokes an isosceles or scalene trapezium.

Conclusion
Based on experiments it can be concluded that the pulsating water jet represents a potentially suitable technology for bone cement removal due to the relatively low pressures necessary for disintegration of the bone cement.Next research will be focused on pulsating water jet disintegration effects on bone cements under changing technological conditions, such as traverse speed, stand-off distance, number of passes, frequency of pulses and nozzle orifice diameter.
The aim is to decrease costs and any potential undesirable clinical impacts connected with the use of this method for the targeted destruction of bone cement during revision surgery.The presented experimental results are relevant for further investigation in order to rationalize the (re)implantation of femoral stems to reduce the thermal effect of bone cement.In such a way the biological potential of bone during surgery can be retained, and by eliminating the negative effects a patient can sooner return to normal life.
Innovation of the exerted surgical procedures with utilization of the water jet in medicine is a vision to the near future with solid foundation in the fierce competitive market environment.Surgical operations using the water jet represent a potential instrument to enhance both surgical operations in favour of patient's life quality improvement and eventually general social and economic impact.In spite of considerable advance in the application of the water jet technology we are aware of the problems related to its application in clinical practice and thus we do believe that by the contribution a possible discussion focused on the field in question shall be initiated.
The results are valid for further investigation in order to rationalize the (re)implantation of femoral stems to reduce the thermal effect of bone cement.In such a way the biological potential of bone during surgery can be retained, and by removing the negative effect a patient can sooner return to normal life.The aim is to decrease costs and potential undesirable clinical impacts connected with their use.This scientific research work is based on the opportunity to apply water jet cutting technology.It comprises the outcomes of exploration and opportunities for the utilization of water jet cutting technology and pulsating water jet for the purpose of the responsible and profitable introduction of such technology in orthopaedic practice.

Figure 1
Figure 1 Scheme of factors affecting the long term success of re-implanted cemented femoral stem after revision surgery

Figure 2Experimental
Figure 2Experimental configuration of high pressure pulsating water jet system

Figure 4 Figure 5
Figure 4 Material removal volume at various values of water pressure for the tested bone cement samples Water pressure MPa Volume of material removal mm 3 Water pressure MPa Volume of material removal mm 3 Technical Gazette 22, 6(2015), 1609-1615

Figure 7
Figure 7 ATB Simplex +T sample after water jetting -surface 3D profile obtained by MicroProf FRT profilometer