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A new technique for cement augmentation of the sliding hip screw in proximal femur fractures
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Clinical Biomechanics 23 (2008) 45–51 www.elsevier.com/locate/clinbiomech
A new technique for cement augmentation of the sliding hip screw in proximal femur fractures Karl K. Stoffel
b
a,b,*
, Toby Leys a, Nikki Damen a, Rochelle L. Nicholls Markus S. Kuster a,b,c
a,b
,
a Fremantle Orthopaedic Unit, The University of Western Australia, Fremantle 6160, Australia Department of Orthopaedic Surgery, The University of Western Australia, Fremantle Hospital, Fremantle 6160, Australia c Department of Orthopaedic Surgery, Kantonsspital St. Gallen, St. Gallen 9027, Switzerland
Received 27 April 2007; accepted 8 August 2007
Abstract Background. Fractures of the osteoporotic proximal femur are a significant source of mortality and morbidity in today’s ageing population. Even with modern fixation techniques such as the sliding hip screw, a certain percentage of fixations will fail due to cut-out of the screw. This study presents a new method for augmenting hip screws with cement to reinforce the fixation. Methods. Unstable pertrochanteric fractures were created in paired osteoporotic cadaver femora (n = 10). The fractures were fixed using either standard fixation techniques (dynamic hip screw), or using a dynamic hip screw augmented with cement. Cement was introduced using a customised jig to guide cement into a region superior to the screw in the femoral head. Cut-out resistance was assessed using a biaxial material testing machine, with loading applied in compression until failure. Findings. The new cement augmentation technique significantly improved the cut-out strength of the fixation (mean 42%; P = 0.032). The failure mechanism for both groups was the same, with failure occurring through compression of the cancellous bone superior to the screw. The mean increase in temperature at the femoral neck was 3.7 °C in augmented bones, which is much lower than values previously reported for polymethylmethacrylate cements. Interpretation. Several benefits with this technique have emerged. The method is technically straightforward. The risk of cement penetration into the joint is reduced, and cement is targetted to the areas of the femoral head where it is most needed. The exothermic reaction is minimised by reducing the volume of cement used. The first clinical results are promising. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Femur; Fracture; Cement; Augmentation; Osteoporosis; Biomechanics
1. Introduction Fractures of the proximal femur are a major source of morbidity and mortality in today’s ageing population (Freeman et al., 2002; Miller, 1978; Roberts and Goldacre, 2003). To prevent problems associated with prolonged recumbency, operative treatment is the preferred manage* Corresponding author. Address: Department of Orthopaedic Surgery, The University of Western Australia, Fremantle Hospital, Fremantle 6160, Australia. E-mail address: nkstoff[email protected] (K.K. Stoffel).
0268-0033/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.clinbiomech.2007.08.014
ment for these fractures, except in cases where medical co-morbidity prevents surgery (Lorich et al., 2004). The fixed angle sliding hip screw implant is the gold standard in management of intertrochanteric proximal femur fractures, and these implants generally have a high success rate (Dodds and Baumgaertner, 2004; Doppelt, 1980; Schumpelick and Jantzen, 1955). However, some mechanical failures are still reported in the literature, and these failures have been associated with bone quality, adequacy of reduction, and the location of the screw within the femoral head (Goodman et al., 1998; Kaufer, 1980). The primary mode of failure is screw cut-out, in which the
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implant is screwed securely to the femoral shaft but the femoral head collapses onto the screw under load; in extreme cases, the screw may break through the femoral head cortex (Simpson et al., 1989). To increase the load at which screw cut-out occurs, osteoporotic bone can be reinforced by injection of bone cement (Heini et al., 2004; Kramer et al., 2000; Mermelstein et al., 1996). Cement is normally applied through the drill hole created for insertion of the hip screw (Kramer et al., 2000). This technique has some disadvantages however. The guide-wire for the screw can puncture the cortex of the femoral head and cement may subsequently leak into the hip joint. Secondly, a considerable amount of cement is inserted into the drill hole, and this cement is not localised to the region of the femoral head which has to withstand the most loading (the region superior to the hip screw). Depending on the volume injected into the femoral head, bone cement can cause osteonecrosis and thermal damage to the cartilage due to the high temperatures generated as the cement sets (Lu et al., 2002). Furthermore, in the case of a future joint replacement being required, removal of the cemented hip screw is surgically demanding. Reducing both the possibility of leakage and the volume of cement that needs to be injected, and guiding cement to the area where the screw will potentially cut-out, is therefore desirable. This study presents a new technique for augmenting the fixation of a sliding hip screw with bone cement. Cement is introduced into the femoral head using a customised jig manufactured using readily-available fixation devices and materials. We further present an algorithm to indicate when cement augmentation is necessary, and present the first clinical cases undertaken using our new technique.
After fixation, the hip screws in Group B were augmented with cement using a new technique developed at our laboratory. A customised stainless steel jig was attached to the DHS plate using the compression screw (Fig. 1). The jig acted as a drill guide, enabling creation of two 4.5 mm holes drilled parallel to the lag screw, centred 2 mm above the hip screw and 15 mm apart. These holes penetrated into the femoral head to a depth equal with the end of the hip screw (within 5–10 mm of the subcortical bone). These holes allowed targeting of cement to the region superior to the lag screw (Fig. 2). The cement used was a very low viscosity radiopaque polymethylmethacrylate (PMMA) cement typically used in vertebroplasty (Heini et al., 2004) (OsteoFirm; William A. Cook Australia, Brisbane, Australia). The cement was mixed according to the manufacturer’s instructions and drawn up into two standard 5 ml syringes (Becton Dickin-
2. Methods Ten femoral heads were harvested from five fresh-frozen female cadavers aged between 67 and 87 years (median age 76 years, standard deviation 4.2). To exclude any pre-existing disease or trauma and to measure specimen bone mineral density (BMD), radiographs of the proximal femurs were taken. BMD was measured by dual-energy X-ray absorptiometry using a Hologic QDR-4500A densitometer (Hologic Inc., Waltham, MA, USA). The specimens were wrapped in saline soaked cloth and stored at 20 °C in sealed plastic bags, and thawed for 24 h prior to testing. The specimens were assigned into two groups (A and B), with pairs of specimens matched according to age and BMD. All biomechanical testing was conducted under the guidelines of The University of Western Australia Research Ethics Committee. The femoral heads (with the neck) were fixed to a metal surrogate of the femoral shaft using a 135° dynamic hip screw (DHS; Synthes, Sydney, Australia). A pertrochanteric fracture pattern with loss of the medial support (AO classification 31-A1) was simulated in each specimen. The DHS was applied in the standard manner for both groups.
Fig. 1. Customised jig attached to the DHS plate for targeted introduction of cement into the femoral head. The set-up is shown modelled using a synthetic femoral head.
Fig. 2. Cross-section of cadaver head showing cement located above the position of the lag screw.
K.K. Stoffel et al. / Clinical Biomechanics 23 (2008) 45–51
son, Singapore; ref 302149). A 4 mm mixing cannula (Unomedical, Sydney, Australia; ref. 500.11.012) was attached to the syringe. The cannula was introduced into the drill holes and cement injected into the femoral head (Fig. 1). The injection was performed by increasing pressure into the cannula while simultaneously pulling it back until the cement started to flow. In order to inject the cement only over the threaded part of the neck screw (which is 20 mm in length), the cement was only injected over a distance of 15 mm – we started to inject the cement at the end of the screw and then augmented the drill hole backwards with cement over a distance of 15 mm. The injected volume was recorded, and radiographs were taken of each augmented femur in order to document the distribution of cement within the bone. To allow non-invasive monitoring of surface temperature at the femoral head, the overlying articular cartilage was removed. Temperature was monitored using a digital laser-light temperature recorder (Raytek Raynger ST, RS 373-8483; Raytek, Sydney, Australia), at a point corresponding to the shortest distance to the underlying bone cement. The sensor has a working range of 20 to 180 °C, with a sensitivity of 1 °C. Measurements were made continuously for 20 min after cement injection. Biomechanical testing was performed using a Zwick Z010 universal material testing machine (Zwick/Roell, Ulm, Germany) and associated software (testXpert V10.11). The test configuration was based on Pauwels’ model of single leg stance, and modified to simulate an osteoporotic stance (Kummer, 1993; Pauwels, 1976). The distal femur was fixed in a block of dental cement (Gladstone 3000, Die Stone Type V, Dentsply International Inc., York, USA), which maintained the femur shaft at an angle of 20° from the vertical within the coronal plane. Load was applied through an acetabular cup acting as a ball and socket joint with a silicon inlay, which was then connected to the testing machine via a cardanic joint (Fig. 3). The femoral head of the specimen was fitted within this cup and load applied using a 5 N preload followed by compression loading at 2 mm/s until failure. The failure point was defined based on that of Heini et al. (2004) as the displacement either when the yield point on the load– displacement curve was reached or after a sudden inflection in the curve due to loss of fixation (fracture at points of fixation or breakage of the implant). Furthermore, the displacement of the screw at failure was the displacement (in mm) at the point where we defined the failure on the load–displacement curve minus the distance (in mm) of the initial non-linear toe region of the load–displacement curve. In this way, we tried to compensate for any other non-failure-load dependent displacement such as initial slack proximal of distal fixations, any displacement under a load when the whole construct was aligned during the set-up, any compression of the remaining cartilage on the femoral head, the silicon in the hemispherical socket and so on. Load–displacement curves and the load at failure were recorded. For each specimen, the pre-test and
47
Fig. 3. Set-up for biomechanical testing of specimens, with the jig attached to the DHS plate and the 4.5 mm drill bit in situ.
post-failure fracture patterns were documented using still photography and radiographs. The data obtained were analysed using the Mann– Whitney U non-parametric two-sample test for independent samples. All statistics were calculated using SPSS 14.0 (SPSS; Chicago, IL), with statistical significance taken as P < 0.05. The relationship between failure-load and BMD was assessed using the Spearman test with the same significance level. 3. Results No difference was found in BMD of the femoral head or neck between the non-augmented (Group A) and non-augmented (Group B) bones (Table 1). All specimens were classified as osteoporotic under the World Health Organisation definition, in which osteoporosis is defined as a bone density of more than 2.5 standard deviations below the peak bone mass. The biomechanical test results are presented in Table 1. The mean total cement volume injected into both drill holes was 3.4 ml (range 2.1–5.2 ml). No leakage of bone cement through the nutrient foramen or into the hip joint was documented. The filling pattern showed a regular and reproducible distribution of cement around the drill hole into the dense cancellous bone at the femoral head of approximately 2 mm depth. After cement augmentation, the maximum surface temperature at the femoral head increased by 3.7 °C from 20.5 to 24.2 °C (P = 0.043). The increase was higher with larger injection volumes. An example of typical load–displacement curves for augmented and non-augmented specimens is shown in
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Table 1 Summary data from non-augmented (Group A) and cement-augmented (Group B) heads in cadaveric bone
Mean BMD (g/cm2) Mean failure load (N) Mean displacement at failure (mm) Baseline temperature (°C) Maximum temperature (°C)
Group A (non-augmented)
Group B (augmented)
Mean
SD
Mean
SD
0.39 1049.6 3.9 n/a n/a
0.05 253.8 0.4 n/a n/a
0.37 1458.4 5.0 20.5 24.2
0.08 226.6 0.7 0.9 2.3
Fig. 4. The linear portion of the curves are shown and the ultimate load marked with an asterix. Use of the modified cement augmentation technique resulted in a significantly higher load at failure for specimens in Group B (1458 ± 227 N) compared to Group A (1049 ± 253 N, P = 0.032). This corresponds to a mean increase in failure-load of 42% for the augmented group. The failure
P
0.841 0.032 0.047 0.043
mechanism for both groups was the same, with the cement–metal construct remaining intact and failure occurring through compression of the cancellous bone superior to the screw. The displacement of the screw at failure was significantly greater in the augmented group (5.0 mm) compared to the non-augmented group (3.9 mm) (P = 0.047). Following failure testing of the cemented specimens, the hip screw could be removed without any further technical problems. A direct correlation of the BMD in the femoral neck and the failure-load was found for Group A (r = 0.9; P = 0.037). This relationship was not significant for the augmented group (r = 0.2; P = 0.747) (Fig. 5).
4. Discussion
Fig. 4. Examples of load–displacement curves, with the ultimate load for augmented and non-augmented specimens indicated by an asterix.
The annual cost of osteoporotic fractures in the UK has been estimated at £942 million (Dolan and Torgerson, 1998), and up to $20 billion in the USA. Of these, approximately 60% are fractures of the hip (Elffors, 1998). The incidence of hip fractures worldwide is expected to quadruple in the next 60 years (Dubey et al., 1998). Hip fractures can cause considerable ongoing disability: one year after fracture, approximately one-third of patients will be deceased (Cooper, 1993), 26% will receive a necessary
Fig. 5. Correlation of femoral neck BMD with failure load for augmented and non-augmented groups.
K.K. Stoffel et al. / Clinical Biomechanics 23 (2008) 45–51
degree of home support, and 53% will be resident in nursing homes (Poor et al., 1995). While efforts have been made to reduce the incidence of osteoporotic fractures through hormonal and pharmacological intervention (Keen, 2007; Martin and Seeman, 2007), exercise programs (Beaudreuil, 2006; Kemmler et al., 2007), and protective devices (Wiener et al., 2002), technical improvements for surgical fixation may also contribute to better clinical outcomes. The mechanical failure rate of the fixed angle sliding hip screw is presently reported as between 4% and 20% (Butt et al., 1995; Hardy et al., 1998; Haynes et al., 1997; Simpson et al., 1989), with the most common failure mode being superior screw cutout (1.1–6.3% (Ahrengart et al., 2002; Bridle et al., 1991; Watson et al., 1998)) with subsequent loss of fixation and varus collapse of the femoral head (Baumgartner and Wahl, 2000). Predictive factors for mechanical failure include adequacy of fracture reduction, failure to centre the hip screw in the femoral head, and tip–apex distance (Baumgaertner et al., 1995; Kim et al., 2001; Stoffel et al., 2006). Previous studies have investigated the use of cement augmentation to reduce the incidence of superior cut-out. Eriksson et al. (2002) compared fixation strength using PMMA cement to that obtained with resorbable calcium phosphate cement, and found PMMA-augmented femurs had a higher load to failure. Bartucci et al. (1985), Claes et al. (1995), Augat et al. (2002) and Szpalski et al. (2004) showed a significantly lower rate of screw cut-out in unstable fractures when the fixation was augmented with cement diffused throughout the femoral head via the lag screw hole. Kramer et al. (2000) showed similar results but noted stability only improved after a cavity in the trabecular bone was created before cement introduction. However, despite these promising results, a number of limitations exist with the conventional cement augmentation technique. The introduction of cement through the lag screw hole provides a less controlled application than that achieved with our method. The importance of targeting cement to the superior femoral head is highlighted by the results for failure-load achieved with our method, in which cement-augmented specimens tolerated a mean of 42% greater compressive load prior to failure. A further technical drawback of injecting cement into the hip lag screw hole is the greater chance of cement penetration into the hip joint, leading to instant thermal necrosis of the cartilage and later to third party wear and rapid onset of osteoarthritis. Moreover, inserting the cement into the hole drilled for the hip screw gives a high chance the cement will leak backward in the tunnel or the fracture gap before insertion of the screw. The presence of cement in the fracture gap has the potential drawback of keeping fragments separated, leading to non-union (Harrington, 1975). The exothermic reaction of the PMMA cements used in many previous studies is well recognised (Baroud et al., 2006; Dunne and Orr, 2002). The magnitude of the exothermic reaction is dose-dependent, in that a greater the volume of cement will generate a greater increase in local
49
temperature. Heini et al. (2004) injected approximately 36 ml of bone cement into the femoral head and femoral neck with a resultant mean temperature increase of 22.1 °C at the femoral neck. A temperature rise of this magnitude is cause for concern with regard to thermal damage to blood vessels and femoral head necrosis. To reduce the risk of exothermic-related thermonecrosis and non-union, Szpalski et al. (2004) used a bispenol-a-glycidyl-dimethacrylate composite with a lower exothermic reaction than normal PMMA. However, this composite cement is quite expensive and not available in every hospital. Our technique is suitable for use with most commercially-available cements, and the mean volume of cement introduced was as low as 4.4 ml, corresponding to a temperature increase of 3.7 °C. Attempts to clinically validate the use of cement augmentation have thusfar been limited. Goodman et al. (1998) reported promising early results in 91 patients treated with SRS cement augmentation, and Szpalski et al. (2004) showed good radiographic results at 6 months in six elderly osteoporotic patients, although neither study employed a control group. No clinical guidelines presently exist in the literature as to when to augment the hip lag screw. We have developed an algorithm to assist surgeons in identifying appropriate cases for augmentation among patients with proximal femur fractures (Fig. 6). Patients with a pertrochanteric femur fracture are typically operated on a traction table. If a stable reduction can be achieved, the fracture will be fixed with a sliding hip screw. However, if reduction cannot be achieved an intramedullary fixation device is employed, such as that used in fixation of subtrochanteric fractures. If, following insertion of the sliding hip screw, the reduction could be maintained and the tip–apex distance is less then 25 mm, the operation is completed. Otherwise the screw can be optionally augmented with the technique described in this article. Using this algorithm, we augmented the neck screw in six of 132 pertrochanteric femur fractures in 2006. We have thus far been able to follow up four patients after 6 months. All
Fig. 6. Algorithm showing the role of cement augmentation of the dynamic hip screw in fracture fixation. PT = pertrochanteric fracture, ST = subtrochanteric fracture, TAD = tip–apex distance, IMN = intramedullary nail.
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showed a healed fracture with no indication of implant failure and no radiological evidence of avascular necrosis of the femoral head. 5. Conclusions This in vitro study showed the effectiveness of reinforcing osteoporotic femoral heads with PMMA using a new augmentation technique which uses drilling above the lag screw. Several benefits with this technique have emerged. Firstly, the method is technically straightforward. Our technique reduces the risk of cement penetration into the joint, and targets the cement to the areas of the femoral head where it is most needed. Finally, the exothermic reaction associated with a high volume of cement is reduced. The cement is not displaced by the introduction of the lag screw, which has previously been shown to introduce cement into the joint, particularly if the guide-wire has penetrated the femoral head. A further benefit of our technique is that the cement is introduced after fracture fixation has been performed. If intraoperative imaging at the end of surgery shows the reduction is unstable, or if the screw position in the femoral head is not ideal, the decision to augment with cement can be made at that stage and performed with little difficulty. The first clinical results are promising. References Ahrengart, L., Tornkvist, H., Fornander, P., et al., 2002. A randomized study of the compression hip screw and Gamma nail in 426 fractures. Clin. Orthop. Relat. Res. (401), 209–222. Augat, P., Rapp, S., Claes, L., 2002. A modified hip screw incorporating injected cement for the fixation of osteoporotic trochanteric fractures. J. Orthop. Trauma 16 (5), 311–316. Baroud, G., Swanson, T., Steffen, T., 2006. Setting properties of four acrylic and two calcium–phosphate cements used in vertebroplasty. J. Long Term Eff. Med. Implant. 16 (1), 51–519. Bartucci, E.J., Gonzalez, M.H., Cooperman, D.R., et al., 1985. The effect of adjunctive methylmethacrylate on failures of fixation and function in patients with intertrochanteric fractures and osteoporosis. J. Bone Joint Surg. Am. 67 (7), 1094–1107. Baumgaertner, M.R., Curtin, S.L., Lindskog, D.M., Keggi, J.M., 1995. The value of the tip–apex distance in predicting failure of fixation of peritrochanteric fractures of the hip. J. Bone Joint Surg. Am. 77 (7), 1058–1064. Baumgaertner, M.R., Wahl, C.M., 2000. Trochanteric fractures. Management of fractures. In: Obrant, K. (Ed.), Severely Osteoporotic Bone. Springer, London, pp. 146–162. Beaudreuil, J., 2006. Nonpharmacological treatments for osteoporosis. Ann. Readapt. Med. Phys. 49 (8), 581–588. Bridle, S.H., Patel, A.D., Bircher, M., Calvert, P.T., 1991. Fixation of intertrochanteric fractures of the femur. A randomised prospective comparison of the gamma nail and the dynamic hip screw. J. Bone Joint Surg. Brit. 73 (2), 330–334. Butt, M.S., Krikler, S.J., Nafie, S., Ali, M.S., 1995. Comparison of dynamic hip screw and gamma nail: a prospective, randomized, controlled trial. Injury 26 (9), 615–618. Claes, L., Becker, C., Simnacher, M., Hoellen, I., 1995. Improvement in the primary stability of the dynamic hip screw osteosynthesis in unstable, pertrochanteric femoral fractures of osteoporotic bones by a new glass ionomer cement. Unfallchirurg 98 (3), 118–123.
Cooper, C., 1993. Epidemiology and public health impact of osteoporosis. Bailliere. Clin. Rheum. 7 (3), 459–477. Dodds, S.D., Baumgaertner, M.R., 2004. The sliding hip screw. Curr. Opin. Orthop. 15, 12–17. Dolan, P., Torgerson, D.J., 1998. The cost of treating osteoporotic fractures in the United Kingdom female population. Osteoporos. Int. 8 (6), 611–617. Doppelt, S.H., 1980. The sliding compression screw-today’s best answer for stabilization of intertrochanteric hip fractures. Orthop. Clin. North Am. 11 (3), 507–523. Dubey, A., Koval, K.J., Zuckerman, J.D., 1998. Hip fracture prevention: a review. Am. J. Orthop. 27 (6), 407–412. Dunne, N.J., Orr, J.F., 2002. Curing characteristics of acrylic bone cement. J. Mater. Sci. Mater. Med. 13 (1), 17–22. Elffors, L., 1998. Are osteoporotic fractures due to osteoporosis? Impacts of a frailty pandemic in an aging world. Aging (Milano) 10 (3), 191–204. Eriksson, F., Mattsson, P., Larsson, S., 2002. The effect of augmentation with resorbable or conventional bone cement on the holding strength for femoral neck fracture devices. J. Orthop. Trauma 16 (5), 302–310. Freeman, C., Todd, C., Camilleri-Ferrante, C., et al., 2002. Quality improvement for patients with hip fracture: experience from a multisite audit. Qual. Saf. Health Care 11 (3), 239–245. Goodman, S.B., Bauer, T.W., Carter, D., et al., 1998. Norian SRS cement augmentation in hip fracture treatment. Laboratory and initial clinical results. Clin. Orthop. Relat. Res. (348), 42–50. Hardy, D.C., Descamps, P.Y., Krallis, P., et al., 1998. Use of an intramedullary hip-screw compared with a compression hip-screw with a plate for intertrochanteric femoral fractures. A prospective, randomized study of one hundred patients. J. Bone Joint Surg. Am. 80 (5), 618–630. Harrington, K.D., 1975. The use of methylmethacrylate as an adjunct in the internal fixation of unstable comminuted intertrochanteric fractures in osteoporotic patients. J. Bone Joint Surg. Am. 57 (6), 744–750. Haynes, R.C., Poll, R.G., Miles, A.W., Weston, R.B., 1997. Failure of femoral head fixation: a cadaveric analysis of lag screw cut-out with the gamma locking nail and AO dynamic hip screw. Injury 28 (5–6), 337–341. Heini, P.F., Franz, T., Fankhauser, C., et al., 2004. Femoroplastyaugmentation of mechanical properties in the osteoporotic proximal femur: a biomechanical investigation of PMMA reinforcement in cadaver bones. Clin. Biomech. 19 (5), 506–512. Kaufer, H., 1980. Mechanics of the treatment of hip injuries. Clin. Orthop. Relat. Res. (146), 53–61. Keen, R., 2007. Osteoporosis: strategies for prevention and management. Best Pract. Res. Clin. Rheum. 21 (1), 109–122. Kemmler, W., Engelke, K., von Stengel, S., et al., 2007. Long-term fouryear exercise has a positive effect on menopausal risk factors: the Erlangen fitness osteoporosis prevention study. J. Strength Cond. Res. 21 (1), 232–239. Kim, W.Y., Han, C.H., Park, J.I., Kim, J.Y., 2001. Failure of intertrochanteric fracture fixation with a dynamic hip screw in relation to preoperative fracture stability and osteoporosis. Int. Orthop. 25 (6), 360– 362. Kramer, A., Angst, M., Gasser, B., Ganz, R., 2000. [Increasing bone screw anchoring in the femur head by cement administration via the implant – a biomechanical study]. Z. Orthop. Ihre Grenzgeb. 138 (5), 464–469. Kummer, B., 1993. Is the Pauwels’ theory of hip biomechanics still valid? A critical analysis, based on modern methods. Ann. Anat. 175 (3), 203–210. Lorich, D.G., Geller, D.S., Nielson, J.H., 2004. Osteoporotic pertrochanteric hip fractures: management and current controversies. Instr. Course Lect. 53, 441–454. Lu, J.X., Huang, Z.W., Tropiano, P., et al., 2002. Human biological reactions at the interface between bone tissue and polymethylmethacrylate cement. J. Mater. Sci. Mater. Med. 13 (8), 803–809. Martin, T.J., Seeman, E., 2007. New mechanisms and targets in the treatment of bone fragility. Clin. Sci. (Lond.) 112 (2), 77–91.
K.K. Stoffel et al. / Clinical Biomechanics 23 (2008) 45–51 Mermelstein, L.E., Chow, L.C., Friedman, C., Crisco III, J.J., 1996. The reinforcement of cancellous bone screws with calcium phosphate cement. J. Orthop. Trauma 10 (1), 15–20. Miller, C.W., 1978. Survival and ambulation following hip fracture. J. Bone Joint Surg. Am. 60 (7), 930–934. Pauwels, F., 1976. Biomechanics of the Normal and Diseased Hip: Theoretical Foundation, Technique, and Results of Treatment: An atlas. Springer, Berlin Heidelberg, New York. Poor, G., Atkinson, E.J., Lewallen, D.G., et al., 1995. Age-related hip fractures in men: clinical spectrum and short-term outcomes. Osteoporos. Int. 5 (6), 419–426. Roberts, S.E., Goldacre, M.J., 2003. Time trends and demography of mortality after fractured neck of femur in an English population, 1968–98: database study. Brit. Med. J. 327 (7418), 771–775. Schumpelick, W., Jantzen, P.M., 1955. A new principle in the operative treatment of trochanteric fractures of the femur. J. Bone Joint Surg. Am. 37-A (4), 693–698.
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Simpson, A.H., Varty, K., Dodd, C.A., 1989. Sliding hip screws: modes of failure. Injury 20 (4), 227–231. Stoffel, K., Lim, T., Billik, B., 2006. An analysis of the predictive factors of failure of the sliding hip screw fixation of fractures of the proximal femur. MOA-APOA Trauma Section Combined Meeting, Kuala Lumpur. Szpalski, M., Descamps, P.Y., Hayez, J., et al., 2004. Prevention of hip lag screw cut-out by cement augmentation: description of a new technique and preliminary clinical results. J. Orthop. Trauma 18 (1), 34–40. Watson, J.T., Moed, B.R., Cramer, K.E., Karges, D.E., 1998. Comparison of the compression hip screw with the Medoff sliding plate for intertrochanteric fractures. Clin. Orthop. Relat. Res. (348), 79–86. Wiener, S.L., Andersson, G.B., Nyhus, L.M., Czech, J., 2002. Force reduction by an external hip protector on the human hip after falls. Clin. Orthop. Relat. Res. (398), 157–168.
Clinical Biomechanics 23 (2008) 45–51 www.elsevier.com/locate/clinbiomech
A new technique for cement augmentation of the sliding hip screw in proximal femur fractures Karl K. Stoffel
b
a,b,*
, Toby Leys a, Nikki Damen a, Rochelle L. Nicholls Markus S. Kuster a,b,c
a,b
,
a Fremantle Orthopaedic Unit, The University of Western Australia, Fremantle 6160, Australia Department of Orthopaedic Surgery, The University of Western Australia, Fremantle Hospital, Fremantle 6160, Australia c Department of Orthopaedic Surgery, Kantonsspital St. Gallen, St. Gallen 9027, Switzerland
Received 27 April 2007; accepted 8 August 2007
Abstract Background. Fractures of the osteoporotic proximal femur are a significant source of mortality and morbidity in today’s ageing population. Even with modern fixation techniques such as the sliding hip screw, a certain percentage of fixations will fail due to cut-out of the screw. This study presents a new method for augmenting hip screws with cement to reinforce the fixation. Methods. Unstable pertrochanteric fractures were created in paired osteoporotic cadaver femora (n = 10). The fractures were fixed using either standard fixation techniques (dynamic hip screw), or using a dynamic hip screw augmented with cement. Cement was introduced using a customised jig to guide cement into a region superior to the screw in the femoral head. Cut-out resistance was assessed using a biaxial material testing machine, with loading applied in compression until failure. Findings. The new cement augmentation technique significantly improved the cut-out strength of the fixation (mean 42%; P = 0.032). The failure mechanism for both groups was the same, with failure occurring through compression of the cancellous bone superior to the screw. The mean increase in temperature at the femoral neck was 3.7 °C in augmented bones, which is much lower than values previously reported for polymethylmethacrylate cements. Interpretation. Several benefits with this technique have emerged. The method is technically straightforward. The risk of cement penetration into the joint is reduced, and cement is targetted to the areas of the femoral head where it is most needed. The exothermic reaction is minimised by reducing the volume of cement used. The first clinical results are promising. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Femur; Fracture; Cement; Augmentation; Osteoporosis; Biomechanics
1. Introduction Fractures of the proximal femur are a major source of morbidity and mortality in today’s ageing population (Freeman et al., 2002; Miller, 1978; Roberts and Goldacre, 2003). To prevent problems associated with prolonged recumbency, operative treatment is the preferred manage* Corresponding author. Address: Department of Orthopaedic Surgery, The University of Western Australia, Fremantle Hospital, Fremantle 6160, Australia. E-mail address: nkstoff[email protected] (K.K. Stoffel).
0268-0033/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.clinbiomech.2007.08.014
ment for these fractures, except in cases where medical co-morbidity prevents surgery (Lorich et al., 2004). The fixed angle sliding hip screw implant is the gold standard in management of intertrochanteric proximal femur fractures, and these implants generally have a high success rate (Dodds and Baumgaertner, 2004; Doppelt, 1980; Schumpelick and Jantzen, 1955). However, some mechanical failures are still reported in the literature, and these failures have been associated with bone quality, adequacy of reduction, and the location of the screw within the femoral head (Goodman et al., 1998; Kaufer, 1980). The primary mode of failure is screw cut-out, in which the
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implant is screwed securely to the femoral shaft but the femoral head collapses onto the screw under load; in extreme cases, the screw may break through the femoral head cortex (Simpson et al., 1989). To increase the load at which screw cut-out occurs, osteoporotic bone can be reinforced by injection of bone cement (Heini et al., 2004; Kramer et al., 2000; Mermelstein et al., 1996). Cement is normally applied through the drill hole created for insertion of the hip screw (Kramer et al., 2000). This technique has some disadvantages however. The guide-wire for the screw can puncture the cortex of the femoral head and cement may subsequently leak into the hip joint. Secondly, a considerable amount of cement is inserted into the drill hole, and this cement is not localised to the region of the femoral head which has to withstand the most loading (the region superior to the hip screw). Depending on the volume injected into the femoral head, bone cement can cause osteonecrosis and thermal damage to the cartilage due to the high temperatures generated as the cement sets (Lu et al., 2002). Furthermore, in the case of a future joint replacement being required, removal of the cemented hip screw is surgically demanding. Reducing both the possibility of leakage and the volume of cement that needs to be injected, and guiding cement to the area where the screw will potentially cut-out, is therefore desirable. This study presents a new technique for augmenting the fixation of a sliding hip screw with bone cement. Cement is introduced into the femoral head using a customised jig manufactured using readily-available fixation devices and materials. We further present an algorithm to indicate when cement augmentation is necessary, and present the first clinical cases undertaken using our new technique.
After fixation, the hip screws in Group B were augmented with cement using a new technique developed at our laboratory. A customised stainless steel jig was attached to the DHS plate using the compression screw (Fig. 1). The jig acted as a drill guide, enabling creation of two 4.5 mm holes drilled parallel to the lag screw, centred 2 mm above the hip screw and 15 mm apart. These holes penetrated into the femoral head to a depth equal with the end of the hip screw (within 5–10 mm of the subcortical bone). These holes allowed targeting of cement to the region superior to the lag screw (Fig. 2). The cement used was a very low viscosity radiopaque polymethylmethacrylate (PMMA) cement typically used in vertebroplasty (Heini et al., 2004) (OsteoFirm; William A. Cook Australia, Brisbane, Australia). The cement was mixed according to the manufacturer’s instructions and drawn up into two standard 5 ml syringes (Becton Dickin-
2. Methods Ten femoral heads were harvested from five fresh-frozen female cadavers aged between 67 and 87 years (median age 76 years, standard deviation 4.2). To exclude any pre-existing disease or trauma and to measure specimen bone mineral density (BMD), radiographs of the proximal femurs were taken. BMD was measured by dual-energy X-ray absorptiometry using a Hologic QDR-4500A densitometer (Hologic Inc., Waltham, MA, USA). The specimens were wrapped in saline soaked cloth and stored at 20 °C in sealed plastic bags, and thawed for 24 h prior to testing. The specimens were assigned into two groups (A and B), with pairs of specimens matched according to age and BMD. All biomechanical testing was conducted under the guidelines of The University of Western Australia Research Ethics Committee. The femoral heads (with the neck) were fixed to a metal surrogate of the femoral shaft using a 135° dynamic hip screw (DHS; Synthes, Sydney, Australia). A pertrochanteric fracture pattern with loss of the medial support (AO classification 31-A1) was simulated in each specimen. The DHS was applied in the standard manner for both groups.
Fig. 1. Customised jig attached to the DHS plate for targeted introduction of cement into the femoral head. The set-up is shown modelled using a synthetic femoral head.
Fig. 2. Cross-section of cadaver head showing cement located above the position of the lag screw.
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son, Singapore; ref 302149). A 4 mm mixing cannula (Unomedical, Sydney, Australia; ref. 500.11.012) was attached to the syringe. The cannula was introduced into the drill holes and cement injected into the femoral head (Fig. 1). The injection was performed by increasing pressure into the cannula while simultaneously pulling it back until the cement started to flow. In order to inject the cement only over the threaded part of the neck screw (which is 20 mm in length), the cement was only injected over a distance of 15 mm – we started to inject the cement at the end of the screw and then augmented the drill hole backwards with cement over a distance of 15 mm. The injected volume was recorded, and radiographs were taken of each augmented femur in order to document the distribution of cement within the bone. To allow non-invasive monitoring of surface temperature at the femoral head, the overlying articular cartilage was removed. Temperature was monitored using a digital laser-light temperature recorder (Raytek Raynger ST, RS 373-8483; Raytek, Sydney, Australia), at a point corresponding to the shortest distance to the underlying bone cement. The sensor has a working range of 20 to 180 °C, with a sensitivity of 1 °C. Measurements were made continuously for 20 min after cement injection. Biomechanical testing was performed using a Zwick Z010 universal material testing machine (Zwick/Roell, Ulm, Germany) and associated software (testXpert V10.11). The test configuration was based on Pauwels’ model of single leg stance, and modified to simulate an osteoporotic stance (Kummer, 1993; Pauwels, 1976). The distal femur was fixed in a block of dental cement (Gladstone 3000, Die Stone Type V, Dentsply International Inc., York, USA), which maintained the femur shaft at an angle of 20° from the vertical within the coronal plane. Load was applied through an acetabular cup acting as a ball and socket joint with a silicon inlay, which was then connected to the testing machine via a cardanic joint (Fig. 3). The femoral head of the specimen was fitted within this cup and load applied using a 5 N preload followed by compression loading at 2 mm/s until failure. The failure point was defined based on that of Heini et al. (2004) as the displacement either when the yield point on the load– displacement curve was reached or after a sudden inflection in the curve due to loss of fixation (fracture at points of fixation or breakage of the implant). Furthermore, the displacement of the screw at failure was the displacement (in mm) at the point where we defined the failure on the load–displacement curve minus the distance (in mm) of the initial non-linear toe region of the load–displacement curve. In this way, we tried to compensate for any other non-failure-load dependent displacement such as initial slack proximal of distal fixations, any displacement under a load when the whole construct was aligned during the set-up, any compression of the remaining cartilage on the femoral head, the silicon in the hemispherical socket and so on. Load–displacement curves and the load at failure were recorded. For each specimen, the pre-test and
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Fig. 3. Set-up for biomechanical testing of specimens, with the jig attached to the DHS plate and the 4.5 mm drill bit in situ.
post-failure fracture patterns were documented using still photography and radiographs. The data obtained were analysed using the Mann– Whitney U non-parametric two-sample test for independent samples. All statistics were calculated using SPSS 14.0 (SPSS; Chicago, IL), with statistical significance taken as P < 0.05. The relationship between failure-load and BMD was assessed using the Spearman test with the same significance level. 3. Results No difference was found in BMD of the femoral head or neck between the non-augmented (Group A) and non-augmented (Group B) bones (Table 1). All specimens were classified as osteoporotic under the World Health Organisation definition, in which osteoporosis is defined as a bone density of more than 2.5 standard deviations below the peak bone mass. The biomechanical test results are presented in Table 1. The mean total cement volume injected into both drill holes was 3.4 ml (range 2.1–5.2 ml). No leakage of bone cement through the nutrient foramen or into the hip joint was documented. The filling pattern showed a regular and reproducible distribution of cement around the drill hole into the dense cancellous bone at the femoral head of approximately 2 mm depth. After cement augmentation, the maximum surface temperature at the femoral head increased by 3.7 °C from 20.5 to 24.2 °C (P = 0.043). The increase was higher with larger injection volumes. An example of typical load–displacement curves for augmented and non-augmented specimens is shown in
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Table 1 Summary data from non-augmented (Group A) and cement-augmented (Group B) heads in cadaveric bone
Mean BMD (g/cm2) Mean failure load (N) Mean displacement at failure (mm) Baseline temperature (°C) Maximum temperature (°C)
Group A (non-augmented)
Group B (augmented)
Mean
SD
Mean
SD
0.39 1049.6 3.9 n/a n/a
0.05 253.8 0.4 n/a n/a
0.37 1458.4 5.0 20.5 24.2
0.08 226.6 0.7 0.9 2.3
Fig. 4. The linear portion of the curves are shown and the ultimate load marked with an asterix. Use of the modified cement augmentation technique resulted in a significantly higher load at failure for specimens in Group B (1458 ± 227 N) compared to Group A (1049 ± 253 N, P = 0.032). This corresponds to a mean increase in failure-load of 42% for the augmented group. The failure
P
0.841 0.032 0.047 0.043
mechanism for both groups was the same, with the cement–metal construct remaining intact and failure occurring through compression of the cancellous bone superior to the screw. The displacement of the screw at failure was significantly greater in the augmented group (5.0 mm) compared to the non-augmented group (3.9 mm) (P = 0.047). Following failure testing of the cemented specimens, the hip screw could be removed without any further technical problems. A direct correlation of the BMD in the femoral neck and the failure-load was found for Group A (r = 0.9; P = 0.037). This relationship was not significant for the augmented group (r = 0.2; P = 0.747) (Fig. 5).
4. Discussion
Fig. 4. Examples of load–displacement curves, with the ultimate load for augmented and non-augmented specimens indicated by an asterix.
The annual cost of osteoporotic fractures in the UK has been estimated at £942 million (Dolan and Torgerson, 1998), and up to $20 billion in the USA. Of these, approximately 60% are fractures of the hip (Elffors, 1998). The incidence of hip fractures worldwide is expected to quadruple in the next 60 years (Dubey et al., 1998). Hip fractures can cause considerable ongoing disability: one year after fracture, approximately one-third of patients will be deceased (Cooper, 1993), 26% will receive a necessary
Fig. 5. Correlation of femoral neck BMD with failure load for augmented and non-augmented groups.
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degree of home support, and 53% will be resident in nursing homes (Poor et al., 1995). While efforts have been made to reduce the incidence of osteoporotic fractures through hormonal and pharmacological intervention (Keen, 2007; Martin and Seeman, 2007), exercise programs (Beaudreuil, 2006; Kemmler et al., 2007), and protective devices (Wiener et al., 2002), technical improvements for surgical fixation may also contribute to better clinical outcomes. The mechanical failure rate of the fixed angle sliding hip screw is presently reported as between 4% and 20% (Butt et al., 1995; Hardy et al., 1998; Haynes et al., 1997; Simpson et al., 1989), with the most common failure mode being superior screw cutout (1.1–6.3% (Ahrengart et al., 2002; Bridle et al., 1991; Watson et al., 1998)) with subsequent loss of fixation and varus collapse of the femoral head (Baumgartner and Wahl, 2000). Predictive factors for mechanical failure include adequacy of fracture reduction, failure to centre the hip screw in the femoral head, and tip–apex distance (Baumgaertner et al., 1995; Kim et al., 2001; Stoffel et al., 2006). Previous studies have investigated the use of cement augmentation to reduce the incidence of superior cut-out. Eriksson et al. (2002) compared fixation strength using PMMA cement to that obtained with resorbable calcium phosphate cement, and found PMMA-augmented femurs had a higher load to failure. Bartucci et al. (1985), Claes et al. (1995), Augat et al. (2002) and Szpalski et al. (2004) showed a significantly lower rate of screw cut-out in unstable fractures when the fixation was augmented with cement diffused throughout the femoral head via the lag screw hole. Kramer et al. (2000) showed similar results but noted stability only improved after a cavity in the trabecular bone was created before cement introduction. However, despite these promising results, a number of limitations exist with the conventional cement augmentation technique. The introduction of cement through the lag screw hole provides a less controlled application than that achieved with our method. The importance of targeting cement to the superior femoral head is highlighted by the results for failure-load achieved with our method, in which cement-augmented specimens tolerated a mean of 42% greater compressive load prior to failure. A further technical drawback of injecting cement into the hip lag screw hole is the greater chance of cement penetration into the hip joint, leading to instant thermal necrosis of the cartilage and later to third party wear and rapid onset of osteoarthritis. Moreover, inserting the cement into the hole drilled for the hip screw gives a high chance the cement will leak backward in the tunnel or the fracture gap before insertion of the screw. The presence of cement in the fracture gap has the potential drawback of keeping fragments separated, leading to non-union (Harrington, 1975). The exothermic reaction of the PMMA cements used in many previous studies is well recognised (Baroud et al., 2006; Dunne and Orr, 2002). The magnitude of the exothermic reaction is dose-dependent, in that a greater the volume of cement will generate a greater increase in local
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temperature. Heini et al. (2004) injected approximately 36 ml of bone cement into the femoral head and femoral neck with a resultant mean temperature increase of 22.1 °C at the femoral neck. A temperature rise of this magnitude is cause for concern with regard to thermal damage to blood vessels and femoral head necrosis. To reduce the risk of exothermic-related thermonecrosis and non-union, Szpalski et al. (2004) used a bispenol-a-glycidyl-dimethacrylate composite with a lower exothermic reaction than normal PMMA. However, this composite cement is quite expensive and not available in every hospital. Our technique is suitable for use with most commercially-available cements, and the mean volume of cement introduced was as low as 4.4 ml, corresponding to a temperature increase of 3.7 °C. Attempts to clinically validate the use of cement augmentation have thusfar been limited. Goodman et al. (1998) reported promising early results in 91 patients treated with SRS cement augmentation, and Szpalski et al. (2004) showed good radiographic results at 6 months in six elderly osteoporotic patients, although neither study employed a control group. No clinical guidelines presently exist in the literature as to when to augment the hip lag screw. We have developed an algorithm to assist surgeons in identifying appropriate cases for augmentation among patients with proximal femur fractures (Fig. 6). Patients with a pertrochanteric femur fracture are typically operated on a traction table. If a stable reduction can be achieved, the fracture will be fixed with a sliding hip screw. However, if reduction cannot be achieved an intramedullary fixation device is employed, such as that used in fixation of subtrochanteric fractures. If, following insertion of the sliding hip screw, the reduction could be maintained and the tip–apex distance is less then 25 mm, the operation is completed. Otherwise the screw can be optionally augmented with the technique described in this article. Using this algorithm, we augmented the neck screw in six of 132 pertrochanteric femur fractures in 2006. We have thus far been able to follow up four patients after 6 months. All
Fig. 6. Algorithm showing the role of cement augmentation of the dynamic hip screw in fracture fixation. PT = pertrochanteric fracture, ST = subtrochanteric fracture, TAD = tip–apex distance, IMN = intramedullary nail.
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showed a healed fracture with no indication of implant failure and no radiological evidence of avascular necrosis of the femoral head. 5. Conclusions This in vitro study showed the effectiveness of reinforcing osteoporotic femoral heads with PMMA using a new augmentation technique which uses drilling above the lag screw. Several benefits with this technique have emerged. Firstly, the method is technically straightforward. Our technique reduces the risk of cement penetration into the joint, and targets the cement to the areas of the femoral head where it is most needed. Finally, the exothermic reaction associated with a high volume of cement is reduced. The cement is not displaced by the introduction of the lag screw, which has previously been shown to introduce cement into the joint, particularly if the guide-wire has penetrated the femoral head. A further benefit of our technique is that the cement is introduced after fracture fixation has been performed. If intraoperative imaging at the end of surgery shows the reduction is unstable, or if the screw position in the femoral head is not ideal, the decision to augment with cement can be made at that stage and performed with little difficulty. The first clinical results are promising. References Ahrengart, L., Tornkvist, H., Fornander, P., et al., 2002. A randomized study of the compression hip screw and Gamma nail in 426 fractures. Clin. Orthop. Relat. Res. (401), 209–222. Augat, P., Rapp, S., Claes, L., 2002. A modified hip screw incorporating injected cement for the fixation of osteoporotic trochanteric fractures. J. Orthop. Trauma 16 (5), 311–316. Baroud, G., Swanson, T., Steffen, T., 2006. Setting properties of four acrylic and two calcium–phosphate cements used in vertebroplasty. J. Long Term Eff. Med. Implant. 16 (1), 51–519. Bartucci, E.J., Gonzalez, M.H., Cooperman, D.R., et al., 1985. The effect of adjunctive methylmethacrylate on failures of fixation and function in patients with intertrochanteric fractures and osteoporosis. J. Bone Joint Surg. Am. 67 (7), 1094–1107. Baumgaertner, M.R., Curtin, S.L., Lindskog, D.M., Keggi, J.M., 1995. The value of the tip–apex distance in predicting failure of fixation of peritrochanteric fractures of the hip. J. Bone Joint Surg. Am. 77 (7), 1058–1064. Baumgaertner, M.R., Wahl, C.M., 2000. Trochanteric fractures. Management of fractures. In: Obrant, K. (Ed.), Severely Osteoporotic Bone. Springer, London, pp. 146–162. Beaudreuil, J., 2006. Nonpharmacological treatments for osteoporosis. Ann. Readapt. Med. Phys. 49 (8), 581–588. Bridle, S.H., Patel, A.D., Bircher, M., Calvert, P.T., 1991. Fixation of intertrochanteric fractures of the femur. A randomised prospective comparison of the gamma nail and the dynamic hip screw. J. Bone Joint Surg. Brit. 73 (2), 330–334. Butt, M.S., Krikler, S.J., Nafie, S., Ali, M.S., 1995. Comparison of dynamic hip screw and gamma nail: a prospective, randomized, controlled trial. Injury 26 (9), 615–618. Claes, L., Becker, C., Simnacher, M., Hoellen, I., 1995. Improvement in the primary stability of the dynamic hip screw osteosynthesis in unstable, pertrochanteric femoral fractures of osteoporotic bones by a new glass ionomer cement. Unfallchirurg 98 (3), 118–123.
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Simpson, A.H., Varty, K., Dodd, C.A., 1989. Sliding hip screws: modes of failure. Injury 20 (4), 227–231. Stoffel, K., Lim, T., Billik, B., 2006. An analysis of the predictive factors of failure of the sliding hip screw fixation of fractures of the proximal femur. MOA-APOA Trauma Section Combined Meeting, Kuala Lumpur. Szpalski, M., Descamps, P.Y., Hayez, J., et al., 2004. Prevention of hip lag screw cut-out by cement augmentation: description of a new technique and preliminary clinical results. J. Orthop. Trauma 18 (1), 34–40. Watson, J.T., Moed, B.R., Cramer, K.E., Karges, D.E., 1998. Comparison of the compression hip screw with the Medoff sliding plate for intertrochanteric fractures. Clin. Orthop. Relat. Res. (348), 79–86. Wiener, S.L., Andersson, G.B., Nyhus, L.M., Czech, J., 2002. Force reduction by an external hip protector on the human hip after falls. Clin. Orthop. Relat. Res. (398), 157–168.