Femoral Cement Pressurization in Hip Arthroplasty

The Journal of Arthroplasty Vol. 22 No. 6 2007

Femoral Cement Pressurization in Hip Arthroplasty A Comparison of 3 Systems Niall Alasdair Munro, MB ChB, MD, FRCSEd (Tr & Orth),* Malcolm Nicol, MB BS, MRCS,* Sivasubramaniam Selvaraj, MSc,y Sheik Mehboob Hussain, MB BS, FRCS,z and David Farquhar Finlayson, MB ChB, FRCS (Orth)§

Abstract: Cement pressurization is critical to achieving optimal results in cemented arthroplasty of the hip. An in vitro experiment using plastic femoral models (10 per group) was undertaken to measure the pressures developed by 3 cementing systems: the Howmedica Mark 1 (Stryker Howmedica, Limerick, Ireland) and DePuy Cemvac retrograde cementation systems (DePuy CMW, Blackpool, UK), and a novel antegrade system consisting of a 60-mL catheter-tipped syringe and a Miller proximal femoral seal (Zimmer Ltd, Swindon, UK). The mean pressure was higher for the syringe system (161.45 F 28.9 kPa) than the Mark 1 (103.51 F 22.0 kPa) or Cemvac (92.65 F 30.7 kPa) systems ( P = .0001). In addition, fewer cement mantle defects were seen with the syringe system (1, interquartile range [IQR] 1-2) than the Mark 1 (3, IQR 2-4) or Cemvac (3, IQR 1-3) systems ( P = .0256). Key words: total hip arthroplasty, femur, bone cement, surgical technique, pressurization. n 2007 Elsevier Inc. All rights reserved.

Cemented total hip arthroplasty has been performed for over 40 years [1], and cementing techniques have gradually evolved and developed over this time. Modern bthird-generationQ methods conventionally use a distal cement restrictor, retrograde insertion of cement from a gun, and a

proximal seal [2,3]. Many different systems based on these principles are commercially available, although there is only limited data about the comparative effectiveness of such systems [3-5]. Cement pressurization has been shown to be beneficial in both radiologic [6,7] and laboratory [8-11] studies but this notwithstanding, long-term failure still occurs in hip arthroplasty. Although the optimal cement pressure in vivo is unknown, in vitro studies have shown a positive relationship between pressure and cement penetration up to pressures well beyond those likely to be achieved with standard pressurization systems [10]. Clinical studies have shown a high incidence of suboptimal cement mantles and cement-bone interfaces [12-14] even when modern cementing techniques are used, and this does imply the inadequacy of current cementation methods. Any device that improves cement pressurization is therefore attractive, although careful preparation of the femur with pulsatile lavage will be necessary to minimize the risks of embolizing marrow contents [15].

From the *University of Aberdeen, Woodend Hospital, Eday Road, Aberdeen, United Kingdom; yHighlands and Islands Health Research Institute, University of Aberdeen, Beechwood Business Park North, Inverness, United Kingdom; zNinewells Hospital, Dundee, United Kingdom and §Raigmore Hospital, Old Perth Road, Inverness, United Kingdom. Submitted July 23, 2005; accepted September 5, 2006. Benefits or funds were received in partial or total support of the research material described in this article from ScheringPlough Ltd, Welwyn Garden City, UK and DuPuy UK, Blackpool, UK. Reprint requests: Niall Alasdair Munro, MD, FRCS (Tr & Orth), Orthopaedic Research Centre, Woodend Hospital, Eday Road, Aberdeen AB15 6LS, UK. n 2007 Elsevier Inc. All rights reserved. 0883-5403/07/1906-0004$32.00/0 doi:10.1016/j.arth.2006.09.014

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894 The Journal of Arthroplasty Vol. 22 No. 6 September 2007 In our institution, a novel cement introduction system is used, which differs from conventional third-generation devices. This system uses a standard catheter-tipped syringe with an anatomical proximal seal and a distal suction catheter to achieve antegrade rather than retrograde canal filling. This system has the advantage of being simpler and cheaper than standard commercial systems. The clinical results when used in conjunction with a double-tapered polished collarless Exeter stem (Stryker Howmedica, Limerick, Ireland) have been excellent, and 206 hips followed-up for a mean of 11.5 years (range, 10-13 years) revealed no femoral revisions for aseptic loosening [16]. The primary question posed in this in vitro study was whether the mean pressure generated during cement introduction and pressurization using this system differed from that with 2 conventional commercially available retrograde third-generation cementation systems. A secondary question related to the integrity of the cement mantles obtained using the different systems, and whether 1 system was more likely than another to result in the inclusion of air bubble defects.

Material and Methods A synthetic proximal femoral model with a strong plastic cortical shell was selected to withstand the anticipated pressures (Sawbones model 1103-2; Pacific Research Laboratories, Malmf, Sweden). The femoral head and neck were re-

Fig. 1. Experimental setup showing a femoral model held by a clamp and attached to 3 transducers. Cement is being introduced in antegrade fashion using the syringe system.

Fig. 2. A prepared femoral model and the syringe system (comprising a Miller seal, 60-mL syringe with truncated nozzle, and catheter).

moved 1 cm proximal to the lesser trochanter, and the femoral canal was prepared with taper pin reamers and a Charnley curette. The open distal end of the bone was sealed with a 2-cm length of wooden dowel secured with epoxy resin. The internal volume of the prepared specimens was approximately 46 mL. Three holes (each 4.5 mm in diameter) were drilled in the medial surface: below the lesser trochanter, at the anticipated level of the distal tip of the prosthesis, and midway between these. Pressure transducers were threaded into the holes and connected to a personal computer to allow pressure recordings every 0.1 second. The model was held in a clamp at an angle representing that of

Fig. 3. The 7 zones (analogous to Gruen’s zones) used for analysis of the cement mantle.

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Fig. 4. The pressure relationship between the (A) proximal, (B) middle, and (C) distal transducers in a typical example. Cementation in this femur was carried out using the syringe system.

896 The Journal of Arthroplasty Vol. 22 No. 6 September 2007

Fig. 5. Typical traces from the middle transducers of the (A) syringe system, (B) Mark 1 system, (C) Cemvac system, and (D) Mark 1 system but with no proximal seal and bthumb pressurizationQ only.

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Fig. 5. (continued)

a patient undergoing hip arthroplasty in the lateral decubitus position while cementation was being performed (Fig. 1). Three different systems were used to insert cement into the femoral models with the experiment being repeated on 10 models for each of the 3 systems. Ambient temperature was kept as constant as possible. The systems were used alternately to minimize any effect of temperature variation or of a blearning curve.Q The first system represented the method used in our institution (Fig. 2). The plunger was temporarily removed from a 60-mL catheter-tipped syringe (Becton Dickinson, Drogheda, Ireland), and two thirds of the nozzle was cut off to decrease resistance to cement flow. A length of tubing connected to a standard operating room suction source was attached to the remainder of the nozzle. Two 40-g mixes of polymethylmethacrylate cement were hand mixed using a bowl and spatula and then sucked into the syringe approximately 1 minute after commencing mixing. After filling the syringe with cement, a 10F catheter (Pennine Health Care, Derby, UK) was inserted down the femur as far as the distal plug. A Miller seal (Zimmer Ltd, Swindon, UK) was placed inside the femoral canal, allowing the catheter to exit laterally. This is an anatomically shaped soft rubber seal with a central hole, which was originally designed for use as part of the Miller cement insertion system. To fill the canal, we applied suction (65 kPa) to the catheter in the femoral canal and injected the cement from the reassembled syringe through the Miller seal, keeping the syringe pressed tightly against the seal. During the initial phase of cement

filling, the seal was tilted very slightly medially to avoid compressing the suction catheter. The catheter was of sufficient rigidity that it did not collapse under the pressures generated by cement insertion. It was removed by an assistant after the canal had been completely filled, whereas the boperatorQ continued to pressurize the cement. Pressure was maintained until the prosthesis was inserted. The second system used consisted of a standard Howmedica Mark 1 cement gun and insert (Stryker Howmedica), with an Exeter Half Moon Femoral Cement Seal (Stryker Howmedica). Cement was mixed as for the first system and sucked into the insert. The gun nozzle was inserted as far as it would go into the stem, and retrograde filling was performed, gradually removing the gun. The operator’s thumb was placed over the neck to help prevent extrusion of cement. After filling the cavity, the gun nozzle was cut flush with the seal. The device was pressed forcibly against the femoral opening, and additional cement was again forced in throughout the period of pressurization. The third system was the Cemvac system and standard femoral pressurizer (both DePuy CMW, Blackpool, UK). This system allows mixing of the cement within a vacuum chamber, which then doubles as the barrel of a gun insert for introduction of the cement; cement is not, therefore, transferred between containers. The filling technique was otherwise similar to that used for the Mark 1 gun. With all the systems, cement was introduced to the femur 2 minutes after mixing and pressurized until 4.5 minutes. An Exeter V40 Size 1, 37.5-mm offset stem (Stryker Howmedica), was then inserted

898 The Journal of Arthroplasty Vol. 22 No. 6 September 2007 with a wingless distal centralizer, to avoid disruption to the cement mantle. Pressure monitoring was carried out for a total of 10 minutes. Two identical stems were used alternately, giving a chance for cooling to room temperature before use. All procedures were done by a single investigator to minimize variability, and the data measurements were hidden from the investigator during the course of the experiments to avoid bias. Room temperature was recorded for each procedure. A medium-viscosity cement (Palacos R; Schering-Plough Ltd, Welwyn Garden City, England) was used. Pressure at each of the 3 transducers was recorded at 0.1-second intervals. The mean pressure for each transducer from when cement introduction was commenced to removal of the pressurization device and insertion of the stem (150 seconds) was calculated, first for each femur, and then across the 10 femora in each group. The stems were removed after curing of the cement, and the femora were sectioned in the coronal plane using a fine-bladed saw to allow air bubbles to be detected. Bubbles out with the plane of sectioning were identified by transilluminating specimens with a light box and were opened using a blunt instrument. This direct inspection gave a much more reliable detection of defects than radiologic examination. Cement mantles were divided into 7 zones analogous to Gruen’s radiologic zones [17], but extending anterior and posterior to the prosthesis up to the sagittal midline (Fig. 3). Bubbles in each of the 7 zones were measured in their maximum dimension: those overlapping zones were attributed to the zone in which most of the defect lay. Defects of less than 2  1 mm could not be reliably detected and were ignored. Cement mantle thickness was not considered because it was felt that this was related to the stem rather than the cement insertion. Assessment was performed by an observer blinded to which introduction system had been used for each model. Results are expressed as mean F SD. We evaluated the pressures of the 3 different systems by a 2-factor analysis of variance for repeated measurements, the between-subjects factor being the system used and the within-subjects factor the transducer.

P values were Geisser-Greenhouse corrected. The number of Gruen’s zones with defects was compared among the 3 systems by means of the Kruskal-Wallis test, with a correction for ties. Hypothesis tests were 2 sided and carried out at the 5% significance level. All statistics were computed with Stata 8.0 (Stata, College Station, Tex).

Results The mean ambient temperature during the experiments was comparable for the 3 groups, being 23.68C F 1.198C for the syringe group, 23.68C F 1.168C for the Mark 1 group, and 23.88C F 0.758C for the Cemvac group. Because of the relatively high temperature and the tight femoral canal, the stems were introduced at 4.5 minutes; in clinical practice, they are usually inserted at 5 minutes. Characteristic pressure profiles were produced by each of the cement introduction systems. The 3 transducers in each specimen tended to show similar readings to each other during cement pressurization but greater variability during stem insertion and cement curing (Fig. 4A-C). Typical comparative pressure graphs for each of the 3 different systems are shown (Fig. 5A-C). Pressure in a single additional femur (not 1 of the 30 in the main experiment), which was filled using the Howmedica Mark 1 gun system and then thumb pressurized, is also included for comparison (Fig. 5D). All systems showed initial pressures approximating to 0 and a peak of pressure on cement introduction. With the Mark 1 and Cemvac systems, there was a brief but marked fall in pressure while the gun was being removed to allow cutting of the nozzle between the filling and pressurization phases. There was then a very high peak of pressure on stem insertion, with a gradual fall off. This decline occurred more slowly in the distal transducer. Toward the end of the recorded 10-minute period, pressures often became markedly negative. This phenomenon has been noted previously and is presumably due to cement contraction during curing [18].

Table 1. Mean F SD Pressures (kPa) Measured by Each Transducer in the 3 Groups Transducer System

Proximal

Middle

Distal

Mean

Syringe Mark 1 Cemvac

166.93 F 28.0 94.41 F 22.3 74.02 F 17.3

166.15 F 29.0 104.65 F 22.2 103.28 F 33.7

151.26 F 30.0 111.46 F 20.2 100.65 F 31.9

161.45 F 28.9 103.51 F 22.0 92.65 F 30.7

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The mean pressures for each of the transducers in the 3 groups are shown (Table 1). The overall mean pressure for each group (the average of the 3 transducer readings) was therefore 161.45 F 28.9 kPa for the syringe system, 103.51 F 22.0 kPa for the Howmedica Mark 1 system, and 92.65 F 30.7 kPa for the DePuy Cemvac system ( P = .0001). The analysis of variance for repeated measures showed that the interaction between the systems used and the transducers on the mean pressures was also significant ( P = .0011). The median number of zones containing cement mantle defects with the 3 systems were as follows: 1 (interquartile range [IQR], 1-2) for the syringe system, 3 (IQR, 2-4) for the Mark 1 system, and 3 (IQR, 1-3) for the Cemvac system ( P = .0256). Defects were most frequent in zone 4; only 5 femora showed no defect in this zone. Five of the femora showed defects around the proximal end of the centralizer. These had a distinctive appearance, and we suspected that they were due to escape of air from within the centralizer.

Discussion Previous studies have compared the pressures obtained with different retrograde systems [3,5], or with finger packing vs specialized insertion devices [19,20]. Only 1 small study [4] has compared the Miller system with any other systems. It found highest pressures with the Miller system, although it looked only at peak rather than mean pressures and used very small sample numbers, precluding any evaluation of statistical significance. Our study also differs in using the Miller seal with an antegrade mode of filling rather than the retrograde Zimmer injection device. In vitro studies do have limitations and are unable to directly measure cement penetration. However, the flow of cement into cancellous bone will depend (according to Poiseuille’s equation) on the net pressure applied to the cement (ie, the difference in pressures between the cement and any intramedullary blood or fat), as well as on cement viscosity and the radius and depth of the intramedullary spaces. Because all these factors except cement pressure are independent of the cement introduction system, cement pressure is an excellent predictor of the relative flow rates obtained with different systems. Cement flow can be regarded as distance penetrated per unit time, and so total penetration can be predicted reliably by the cement pressure and the time for which that pressure is applied, or alternatively mean pressure

over a fixed period. It is difficult to measure cement pressure in human subjects without damaging the femoral cortex or cement mantle and also to ensure comparable study groups given interpatient variability. Animal models are of dubious validity in evaluating equipment designed specifically for human anatomy. Despite the limitations, therefore, pressurization is probably most practically measured using in vitro techniques. Although the cement was mixed in a different way for the Cemvac system than for the other 2, this was unavoidable because of the Cemvac mixing and injection systems being integral to each other. It is possible that the different mixing technique for this group may have affected the frequency of air bubble defects in the cement mantle, but there is no reason to suspect any effect on pressure. This experiment made no attempt to look at microporosity [21], and the smallest air bubbles, which are most likely to have been due to mixing technique, were excluded from analysis. Although a simple open bowel was used experimentally for cement mixing for the syringe and Mark 1 systems, a vacuum mixing bowl may be used in clinical practice. In this study, the syringe system showed a significantly higher mean pressure than the others in this experiment. The question as to whether these differences are clinically significant inevitably arises. A bdose-effectQ relationship between pressure and component fixation has been suggested up to a threshold of 1 MPa on the basis of in vitro experimental evidence [10]. It is difficult to extrapolate this to the clinical situation, but it seems likely that, if anything, higher pressures will be required to also overcome bleeding pressure and to displace viscous fluid such as blood from trabecular spaces. Optimal pressures may vary from patient to patient and will also depend on cement viscosity and other factors. All systems are likely to generate lower pressures in vivo than in these somewhat idealized experimental conditions, and it seems wise therefore to opt for a system that is capable of generating higher rather than lower pressures. It has been suggested that the high peak pressures generated by stem insertion effectively render any prior differences in pressurization irrelevant [18]. However, penetration is related to time and (inversely) to viscosity as well as pressure. The stem insertion peak is transient (Figs. 4 and 5) and coincides with the period of high viscosity. We agree with most other authors, therefore, that early prolonged pressurization is of relevance to achieving good interdigitation of cement between the trabeculae [11,19,22].

900 The Journal of Arthroplasty Vol. 22 No. 6 September 2007 The higher pressure that was obtained with the syringe system might be explained by a number of design features. The first is that the seal is anatomically designed to fit a few millimeters into the femoral neck, whereas the seals used with the other 2 systems sit over the opening rather than within it. This may be less secure, and it also means that a significant part of any compressive force is directed onto the femoral rim rather than the cement, contributing nothing to pressurization. The second point is that with the syringe system, forces directed onto the plunger to extrude cement will also help to press the syringe, seal, and femur together, helping to avoid leaks and depressurization at these interfaces. With a gun, the dominant hand is occupied largely with pulling a trigger away from the femur and plays little part in maintaining compression at the gun-seal or seal-femur interfaces. The third point is that the small size of a 60-mL syringe and, to a lesser extent, the Mark 1 gun allow a greater mechanical efficiency, with the operator being able to bring much of his body weight to bear above the device, improving compression. This is difficult with the much larger Cemvac gun. An additional feature of the first system is that no adjustment to the injection device is required during the procedure. With both the alternative systems, it is necessary to remove the gun and cut the nozzle after filling the canal and before pressurization. This results in a period of almost 0 pressurization, which will adversely affect the mean pressure. The fact that pressure actually falls to below bleeding pressure means that it may have an effect on the cement-bone interface, which is disproportionate to its effect on mean pressure. Although some trabeculae may drain into the vascular system, allowing any blood to be forced back into the circulation by subsequent pressurization, others are probably effectively bclosed,Q and no amount of subsequent pressurization will therefore improve cement interdigitation into these spaces once bleeding has been allowed to occur. This period of low pressurization is therefore a concern, especially because it occurs during the period of minimum cement viscosity when it is most likely to be displaced. The final point relates to the antegrade nature of filling using the first system. It seems unlikely that this will affect mean pressure markedly, although the extremely short nozzle of an antegrade filling system may play a small part by decreasing resistance to cement flow. It does, however, significantly affect the distribution of pressure. All systems have a period from when the femoral canal

is unpacked to when it is fully filled with cement, and this is an opportunity for bleeding to occur. With a retrograde system, distal filling occurs before proximal, and this favors distal pressurization. The opposite scenario occurs with antegrade filling. Thus, in our experiments, mean pressure was always higher in the proximal than the distal transducer with the antegrade syringe system, but the opposite scenario existed with the 2 retrograde guns. Prosthesis insertion produces more prolonged distal than proximal pressurization [19], so it may be particularly important to optimize proximal compression in the pressurization phase. Canal filling appeared to be achieved more rapidly using a syringe, because it could be filled with a single smooth motion. A gun necessitates multiple strokes on a trigger and intervening periods of relaxation, and this may give more time for bleeding. Against this, however, an antegrade system necessitates the femoral cavity being under partial vacuum conditions during filling because of the suction catheter (Figs. 4 and 5A), and this may increase bleeding and negate the effect of faster filling. The volume of the prepared femoral cavity was relatively constant and perhaps marginally less than average [5]. This meant that the capacity of the different systems was not really put to the test, although in 1 or 2 of the Mark 1 group, the gun did run out of cement toward the end of the pressurization phase. In clinical practice, a 60-mL syringe also occasionally runs out of cement during canal filling and subsequent pressurization. In this scenario, an assistant takes the syringe and refills it using the remaining cement, whereas the operator maintains the pressure on the proximal seal and hole. Insufficient volume within a gun, such as the Mark 1, is a significant problem and effectively means resorting to finger packing additional cement with less than optimal pressurization (Fig. 5D). The high volume of the Cemvac gun and the fact that no cement is wasted because of transferring it between containers mean that this is unlikely to be a problem with this system, and this advantage may have come into play in a highervolume femur. We had been concerned that an antegrade filling system might result in more filling defects. In practice, however, the opposite appears to be the case and it appears that a distal venting catheter is an effective way to remove air from the femur. This accords with our clinical experience. Assuming the femur is prepared with reasonable care, we have not had problems because of occlusion of the tube with debris. Although not specifically recommended by the manufacturers, there may in fact be a

Femoral Cement Pressurization ! Munro et al 901

case for using a distal venting tube even with retrograde guns, especially if the nozzle is either too wide or too short to reach as far as the cement restrictor. Although high pressures and satisfactory cement mantles have been obtained using simple components, there may be room for development of the technique. Development of a purpose-designed syringe with a higher volume would allow for filling of high-volume canals without the occasional need to refill syringes or to use a second syringe. A slightly wider nozzle would further decrease resistance to cement flow and improve pressurization, whereas a rounded plunger handle might make it easier to compress. Nevertheless, even as it stands, the system does appear to show advantages over a number of more conventional commercial retrograde filling systems.

Acknowledgments We are grateful to Schering-Plough Ltd for supplying Palacos R bone cement for use in this experiment and to DePuy UK for supplying the Cemvac systems and for loaning the pressuremonitoring equipment.

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