Biomechanical evaluation of reconstructed lumbosacral spine after total sacrectomy

J Orthop Sci (2002) 7:658–664

Biomechanical evaluation of reconstructed lumbosacral spine after total sacrectomy Hideki Murakami1, Norio Kawahara1, Katsuro Tomita1, Jiro Sakamoto2, and Juhachi Oda2 1 2

Department of Orthopaedic Surgery, Kanazawa University, 13-1 Takaramachi, Kanazawa 920-8641, Japan Department of Human and Mechanical Systems Engineering, Kanazawa University, Kanazawa, Japan

Abstract When a sacral tumor involves the first sacral vertebra, total sacrectomy is necessary. It is mandatory to reconstruct the continuity between the spine and the pelvis after total sacrectomy. In this study, strain and stress on the instruments and the bones were evaluated for two reconstruction methods: a modified Galveston reconstruction (MGR) and a triangular frame reconstruction (TFR). Compressive loading tests were performed using polyurethane vertebral models, and a finite element model of a lumbar spine and pelvis was constructed. Then three-dimensional MGR and TFR models were reconstructed, and finite element analysis was performed to account for the stress on the bones and instruments. With MGR, excessive stress was concentrated at the spinal rod, and there was a strong possibility that the rod between the spine and the pelvis might fail. Although there was no stress concentration on the instruments with TFR, excessive stress on the iliac bone around the sacral rod was more than the yielding stress of the iliac bone. Such stress may cause loosening of the sacral rod from the iliac bone. If the patient were to stand or sit immediately after MGR or TFR, instrumentation failure or loosening might occur. Key words Sacral tumor · Total sacrectomy · Reconstruction

Introduction Among the sacral tumors, aggressive benign bone tumors such as chordoma, giant cell tumor, and chondrosarcoma are the most common. If a malignant tumor or an aggressive benign tumor are excised with a intralesional margin, the likelihood of local recurrence apparently increases, which can lead to fatal disease. Therefore, en bloc sacrectomy with enough tumor-free margin is indicated even though the lumbosacral nerves

Offprint requests to: H. Murakami Received: March 4, 2002 / Accepted: June 27, 2002

and the continuity between the pelvis and spine are sacrificed. However, curative excision of sacral tumors may be considered difficult owing to the anatomical relation between the sacrum and the plexus of lumbosacral nerves and vessels on the one hand and the intrapelvic organs on the other. It is also too difficult to reconstruct the continuity between pelvis and spine because of the weight of the body, which passes across the lumbosacral junction. To reduce local recurrence as much as possible and to obtain a long survival time, Tomita and colleagues developed a surgical classification of sacral tumors and a total or partial (segmental) en bloc sacrectomy technique with a T-saw.21,22 This surgical concept and technique were established on the basis of total en bloc spondylectomy (TES).18–20,23 It is possible to excise the tumor mass with a wide or marginal margin using this technique. If the tumor extends to the first sacral vertebra, total sacrectomy is mandatory. Reconstruction of the pelvic ring and spinal continuity is then difficult. Some authors have previously described such a reconstruction.3,7,13,14,16 One of the methods is a modified Galveston reconstruction (MGR)3,7 using the Galveston method.1,2 Another method is a triangular frame reconstruction (TFR).22 Instrumentation failure or loosening often occurs with both of them, and there has been no mechanical analysis of these reconstructions. In the present study, strain and stress on the instruments and the bones were evaluated for the MGR and the TFR using a model experiment and a finite element analysis. Finite element analysis is one of the effective procedure for evaluating the reconstruction biomechanically in detail.6 This study is the first step in the development of a more appropriate reconstruction after total sacrectomy.

H. Murakami et al.: Reconstruction of total sacrectomy

659

Materials and methods Method of reconstruction The MGR and the TFR were evaluated as models of reconstruction after total sacrectomy. With the MGR, after placing pedicle screws into the L3-5 vertebral bodies and two bilateral iliac screws into the bilateral iliac bone, these screws were connected by two spinal rods. With the TFR, after the placing pedicle screws in the L3-5 vertebral bodies, the spinal column was pulled down (3 cm) and L5 was affixed to the bilateral ilium with sacral rods. Another sacral rod extending into the pelvis was connected to the spinal rod, which was affixed to the pedicle screws of L3-5.

A

Preparation of reconstruction model using polyurethane Total sacrectomy was performed, and the MGR and TFR were done using two polyurethane spine-pelvis models (Sawbones; Pacific Research Laboratories, Vashon, WA, USA). The polyurethane model is consistent among specimens and easier to store and prepare than cadaver or animal specimens.4 Titanium alloy (Ti6Al-4V) instruments were used with the MGR model (Fig. 1A), and the spinal column and pelvis were fixed in the same position as before sacrectomy. After placing six 6.5 mm diameter pedicle screws (Century Medical, Columbus, OH, USA) 40 mm long into the third to fifth lumbar vertebrae and two (10 mm diameter ⫻ 95 mm long and 8 mm diameter ⫻ 40 mm long) bilateral iliac screws (Century Medical) into the two iliac bones, these screws were connected using two 6.35 mm diameter spinal rods. The 10 mm diameter iliac screws were inserted into the anteroinferior iliac spine. Bilateral spinal rods were connected using two crosslinks (Century Medical). Stainless steel instruments were used for the TFR model (Fig. 1B). The spinal column was pulled down vertically, and L5 was fixed to the bilateral ilium with a 6.4 mm diameter sacral rod (Zimmer, Warsaw, IN, USA) extending into the L5 vertebral body. Another 6.4 mm diameter sacral rod extending into the pelvis was connected to the 5 mm diameter spinal rod (Depuy, Cleveland, OH, USA), which was fixed to six 6 mm diameter ⫻ 45 mm long pedicle screws (Depuy, Cleveland, OH) of the third, fourth, and fifth lumbar vertebrae using two inlets (Zimmer). Bilateral spinal rods were connected using two crosslinks (Depuy).

Preparation of testing machine to apply a load Each polyurethane model of the two reconstructions after total sacrectomy was then potted up to its acetabu-

B Fig. 1A,B. Modified Galveston reconstruction (MGR) and triangular frame reconstruction (TFR) structures using polyurethane models. A MGR model. B TFR model

lum in a 25 ⫻ 30 ⫻ 12 cm wooden box using plaster. The testing machine (Fig. 2) for this experiment was made of stainless steel. A vertical compressive load was applied through a ball (8 mm diameter) to the top surface of the third vertebra. An aluminum board 1 mm thick was placed on the L3 vertebra to transmit the load to the vertebra. Strain measurement Strain measurement were carried out for the TFR and MGR structures, for which the polyurethane spine and pelvis models were used. A pure compressive load of 412 N was applied at the center of the L3 vertebra. The strain on several locations of instruments for the MGR and the TFR was measured using strain gauges (Fig. 3). Figure 4 shows the location of the strain gauges. The strain gauges were placed on the posterior aspect of the spinal rods or iliac screw. The load was applied ten times for each model, and a strain value was obtained each time using the measuring instrument (Kyowa, Tokyo, Japan). The average value was taken.

660

H. Murakami et al.: Reconstruction of total sacrectomy

Construction of a finite element model of reconstruction A finite element model of the lumbar spine and pelvis was constructed based on computed tomography (CT) images of the lumbar vertebrae and pelvis of a healthy 28-year-old man. Then, after the sacrum was removed and instrumentation was added, simulating the operations of both MGR and TFR, three-dimensional MGR and TFR models were reconstructed (Fig. 5). The reconstruction was a half model because of the symmetry and contained eight-noded solid elements and sixnoded solid elements. The MGR model consist of 4912 elements and the TFR model 4854 elements. Constant cortical thickness was assumed to be 1.5 mm for the vertebrae and 3 mm for the pelvis. The endplate thickness was assumed to be 1 mm,11 and the thickness of

Fig. 2. Experimental setup of the compressive loading test. A weight is placed on the cross head. The compressive load is then applied vertically to the upper surface of the third lumbar vertebral body by a load punch with a steel ball. The bottom of the pelvis is fixed to a wooden box with plaster rod1 rod2

the annulus fibrosus was assumed to be 10 mm. The constant rod diameter was assumed to be 6 mm, the crosslink diameter 3 mm, and the diameter of the screws was assumed to 6.5 mm for pedicle screws and to 8 mm for iliac screws.

Finite element analysis of reconstruction models A finite element analysis was performed to account for the stress on the bones and instruments. Boundary conditions were that 480 N force was applied vertically to half of the region of the upper surface of the L3 vertebra, and the bottom of the pelvis was fixed. The 480 N load on this model is equivalent to a load of 960 N because of it being a half-model. The load of 960 N was chosen on the basis of it being within the range of 500–

Fig. 3. Strain measurement in the TFR structure under a compressive load. Strain at the instruments is measured in both the MGR and TFR structures. Polyurethane models of the lumber spine and pelvis are used

rod3 rod4 rod5

screw

B

A Fig. 4A,B. Schemes representing locations of the strain gauges. A MGR model. B TFR model. rod1, measuring point on the rod between the pedicle screw of L4 and that of L5; rod2, measuring point on the rod between the pedicle screw of L5 and the iliac screw; screw, measuring point on the neck of

the bottom iliac screw; rod3, measuring point on the rod between the pedicle screw of L3 and that of L4; rod4, measuring point on the rod between the pedicle screw of L4 and that of L5; rod5, measuring point on the rod under the pedicle screw of L5

H. Murakami et al.: Reconstruction of total sacrectomy

2000 N (500 N corresponds to erect standing and 2000 N to light manual labor).12 The instruments for MGR were composed of titanium alloy, and those for TFR were made of titanium alloy and stainless steel. The properties of Young’s modulus and Poisson’s ratio are summarized in Table 1.5,15 MARC and MENTAT (MARC Analysis, Tokyo, Japan) were used for stress analysis.

Results The strains on rods 1 and 2 with the MGR model were 0.2950 ⫻ 10⫺2 and 0.3208 ⫻ 10⫺2, respectively. The strain on the iliac screw neck was 0.0255 ⫻ 10⫺2. Maximum strain was observed on rod 2 (Fig. 6A). With the TFR model, the strains on rods 3, 4, 5 were 0.2316 ⫻ 10⫺2, 0.2704 ⫻ 10⫺2, and 0.2487 ⫻ 10⫺2, respectively (Fig. 6B). In the finite element analysis of the MGR model, a maximum Mises stress of 13.8 MPa at the pelvis was observed in an area of the lateral cortical bone around the inferior iliac screw (Fig. 7A). Concerning the stress of cancellous bone around the iliac screw, the maximum stress was only 4.7 MPa at the both ends of the screw. The maximum stress on L5 was also only 9.6 MPa. The maximum stress (1042 MPa) of the instruments was observed at an area of the spinal rod between the L5 pedicle screw and the iliac screw (Fig. 7B). With the TFR using stainless steel, the maximum stress (118 MPa) at the pelvis was observed at the medial cortical bone around the sacral rod (which was inserted through the L5 vertebra) (Fig. 8A). The stress of L5 was 73 MPa (Fig. 8B). With regard to instruments, maximum stress (229 MPa) was observed at the point of the sacral rod where it inserted into the L5 vertebra between L5 and the iliac bone. At a point of the spinal rod between the L5 and the L4, a high stress of 212 MPa was observed (Fig. 8C). With the TFR using titanium alloy, the area of maximum stress concentration in the pelvis was the same as that for the stainless steel TFR model. The stress (126 MPa) was a little higher than that of the stainless steel TFR model (Fig. 9A). The stress at L5 was 92.8 MPa (Fig. 9B). With regard to instruments, the maximum stress was observed at the same point of the sacral rod (222 MPa) and spinal rod (159 MPa) as for the stainless steel model (Fig. 9C).

Discussion In the case of sacral resection, preservation of the sacroiliac joint has a great impact on the stability between the spine and the pelvis. Gunterberg and Stener8 analyzed pelvic strength after major amputation of the sacrum

661 Table 1. Material properties in the clinical model Material Cortical bone Cancellous bone Bony posterior elements Cartilaginous endplate Annulus fibrosus Nucleus pulposus Instruments (stainless steel) Instruments (titanium alloy)

Young’s modulus (Mpa)

Poisson’s ratio

12 000 100 3 500 24 4.2 1 667 210 000 110 000

0.30 0.20 0.25 0.40 0.45 0.48 0.30 0.30

using cadavers and found that the pelvic ring was weakened by approximately 30% with resection of the sacrum between S1 and S2. This increased to 50% when the resection was 1 cm below the sacral promontory. They concluded that it was safe to allow patients with such resections to stand, bearing their full weights, postoperatively. When a sacral tumor involves the first sacral vertebra, total sacrectomy is necessary. It is mandatory to reconstruct the continuity between the spine and the pelvis after total sacrectomy, but the optimal reconstruction procedure has not been established. There have been no previous studies on the mechanical analysis of the reconstructed spine. In the study presented here, strain and stress on the instruments and bones were evaluated for two reconstruction methods (MGR and TFR). These reconstructions may be the most appropriate reconstruction procedures so far. Generally, the risk of breakage or loosening at the points at which undue stress is concentrated is higher. With the titanium alloy MGR model of this study, maximum Mises stress (13.8 MPa) at the pelvis was observed in an area of the lateral cortical bone around the inferior iliac screw. This stress is less than the yield stress of cortical bone (83 MPa). The maximum stress at cancellous bone around the iliac screw was only 4.7 MPa which is also less than the yield stress of cancellous bone (30 MPa). The maximum stress (9.6 MPa) at L5 was also less than the yield stress. However, for the instruments, high stress (1042 MPa) was observed in an area of the spinal rod between the L5 pedicle screw and the iliac screw. This stress is higher than the yield stress of the titanium alloy (860 MPa). The strain of the spinal rod between the L5 pedicle screw and the iliac screw was also high. If the patient were to stand or sit immediately after the reconstruction, there would be a risk of fracturing the instruments because of high stress in the spinal rods. Breakage of the spinal rod at this juncture has been found clinically.9,10 With the TFR using stainless steel, the maximum stress (229 MPa) of the instruments was observed at the superior aspect of the sacral rod between L5 and the

A,B Fig. 5 Fig.6

B

A Fig. 7

A,B

C Fig. 8

A,B

C Fig. 9

H. Murakami et al.: Reconstruction of total sacrectomy

iliac bone. A stress of 212 MPa was observed at a point in the spinal rod between L5 and L4. Both of these stress concentrations are more than the yield stress of stainless steel (200 MPa). The strain of the spinal rod between L5 and L4 was also greater than that of the spinal rod below the L5 pedicle screw. If the patient were to sit immediately after the stainless steel TFR, failure could occur at those points of the sacral rod or the spinal rod. On the other hand, no particular stress concentration was observed in the TFR instruments made of titanium alloy, suggesting that there is little risk of instrumentation failure. With the TFR using titanium alloy, high stress (126 MPa) was observed at the interface between the pelvis and the upper sacral bar. The stress concentration (92.8 MPa) of L5 was also higher than the yield stress of cortical bone. It is possible that sacral bar loosening occurs because of the stress being higher than the yield stress of cortical bone at the interface with the pelvis and L5 with TFR. With MGR, excessive stress was concentrated at the spinal rod, as all of the compressive load was transmitted to the pelvis by the spinal rods. There is a strong possibility that the rod between the spine and the pelvis might fail under such conditions. In contrast, the compressive load was transmitted to the iliac bone through the anterior sacral rod and spinal rods with TFR. Although there was no stress concentration on the instruments, excessive stress on the iliac bones and the L5 vertebral body around the sacral rod was more than the yielding stress. Such a stress may cause loosening of the sacral rod from the iliac bone or the L5 vertebral 䉳

Fig. 5. Anterior oblique view of finite element mesh in the MGR (A) and TFR (B) models. Pink, orange, red, dark blue, sky blue, olive, and ocher represent cortical bone, cancellous bone, instruments, bony posterior elements, cartilaginous endplate, annulus fibrosus, and nucleus pulposus, respectively. The finite element mesh indicating the instruments are added to the geometrically constructed finite element mesh of the vertebrae and the pelvis, simulating the operations of both the MGR and TFR Fig. 6A,B. Strain at each instrument point in the MGR and TFR models. A Titanium alloy MGR model. B Stainless steel TFR model. rod1, measuring point on the rod between the pedicle screw of L4 and that of L5; rod2, measuring point on the rod between the pedicle screw of L5 and the iliac screw; screw, measuring point on the neck of the bottom iliac screw; rod3, measuring point on the rod between the pedicle screw of L3 and that of L4; rod4, measuring point on the rod between the pedicle screw of L4 and that of L5; rod5, measuring point on the rod under the pedicle screw of L5 Fig. 7A,B. Finite element analysis of the titanium alloy MGR model. The right half of the model is shown. Color scale to the right represents the stress magnitude. A Posterior view of the MGR structure. P, maximum stress observed on the pelvis under compressive loading; V, maximum stress observed on the vertebrae. B Posterior view of instruments applied to the MGR structure. The maximum stress is observed at a point on

663

body. Further study to reduce the stress concentration at that point is necessary. Moreover, only a pure compressive load was applied in this study. The actual bare vectors applied to the spine clinically are more complicated. Further mechanical evaluation of the reconstruction regarding flexion, extension, lateral bending, and axial torsion is needed. Postoperative management after total sacrectomy has depended on each surgeon’s decision because of the lack of biomechanical analysis for each reconstruction. Generally, patients are allowed to begin walking 2–3 months after total sacrectomy. At our institute, patients generally walks about 7 weeks after surgery.17 If the patient were to stand or sit immediately after MGR or TFR, instrumentation failure or loosening might result. The present results suggest that patients should avoid total weight-bearing; they require bed rest until the grafted bone is fused. Every effort should be made to improve the procedure for spinal reconstruction after a total sacrectomy. This study represents a step toward establishing an ideal method of reconstruction.

Conclusions With MGR, excessive stress was concentrated at the spinal rod. There is a strong possibility that the rod between the spine and the pelvis might fail under such conditions. Although there was no stress concentration on the instruments with TFR, excessive stress on the iliac bones around the sacral rod was observed to be

the spinal rod between the pedicle screw of the fifth lumbar vertebra and the iliac screw (I) Fig. 8A–C. Finite element analysis of the stainless steel TFR model. The right half of the model is shown. Color scale to the right represents the magnitude of the stress. A Anterior view of the stainless steel TFR structure. Excessive stress above the yield stress of cortical bone occurs at the interface between the pelvis and the upper sacral rod (P). B Lateral view of lumbar vertebrae in the TFR structure. V, maximum stress observed on the vertebrae under compressive loading. C Lateral oblique view of instruments applied to the TFR structure. The maximum stress on the instruments is observed on the sacral rod under compressive loading (I). S, maximum stress observed on the spinal rods Fig. 9A–C. Finite element analysis of the titanium alloy TFR model. The right half of the model is shown. Color scale to the right represents the magnitude of the stress. A Anterior view of the titanium alloy TFR structure. Excessive stress above the yield stress of cortical bone occurs at the interface between the pelvis and the upper sacral rod (P). B Lateral view of lumbar vertebrae in the TFR structure. V, maximum stress observed on the vertebrae under compressive loading. C Lateral oblique view of instruments applied to the TFR structure. Maximum stress on the instruments is observed on the sacral rod under compressive loading (I). S, maximum stress observed on the spinal rods

664

more than the yield stress of the iliac bone. This could cause loosening of the sacral rod in the iliac bone. Acknowledgments. The authors thank Dr. William C. Hutton of the Emory Spine Center for manuscript review and helpful suggestions, and Naoto Takeuchi, M.D., Katsuyuki Funaki, Takumi Sakai, and Mayumi Chida for their invaluable assistance.

References 1. Allen BL Jr, Ferguson RL. The Galveston technique for L-rod instrumentation of the scoliotic spine. Spine 1982;7:276–84. 2. Allen BL Jr, Ferguson RL. A pictorial guide to the Galveston LRI pelvic fixation technique. Contemp Orthop 1983;7:51– 61. 3. Blatter G, Halter Ward EG, Ruflin G, et al. The problem of stabilization after sacrectomy. Arch Orthop Trauma Surg 1994; 114:40–2. 4. Dick JC, Zdeblick TA, Bartel BD, et al. Mechanical evaluation of cross-link designs in rigid pedicle screw systems. Spine 1997; 22:370–5. 5. Goel VK, Lim TH, Gilbertson LG. Clinically relevant finite element models of a ligamentous lumbar motion segment. Semin Spine Surg 1993;5:29–41. 6. Goel VK, Gilbertson LG. Applications of the finite element method to thoracolumbar spine research-past, present, and future. Spine 1995;20:1719–27. 7. Gokaslan ZL, Romsdahl MM, Kroll SS, et al. Total sacrectomy and Galveston L-rod reconstruction for malignant neoplasms. J Neurosurg 1997;87:781–7. 8. Gunterberg B, Stener B. Pelvic strength after major amputation of the sacrum. Acta Orthop Scand 1976;47:635–42 9. Jackson RJ, Gokaslan ZL. Spinal-pelvic fixation in patients with lumbosacral neoplasm. J Neurosurg 2000;92:61–70.

H. Murakami et al.: Reconstruction of total sacrectomy 10. Kuroki H, Tajima N, Kubo S, et al. Instrument failure after total sacrectomy and reconstruction of the sacrum. Nishinihonsekituikenkyukaisi (Journal of the Western Japan Spine Meeting) 1998;24:209–12. (in Japanese). 11. Lu YM, Hutton WC, Gharpuray VM. Can variations in intervertebral disc height affect the mechanical function of the disc? Spine 1996;21:2208–17. 12. Nachemson A, Elfstrom G. Intravital dynamic pressure measurements in lumbar disc: a study of common movements, maneuvers and exercises. Scand J Rehabil Med 1970;1:1–40. 13. Santi MD, Mitsunaga MM, Lockett JL. Total sacrectomy for a giant sacral schwannoma. Clin Orthop 1993;194:285–9. 14. Shikata J, Yamamuro T, Kotoura Y, et al. Total sacrectomy and reconstruction for primary tumors. J Bone Joint Surg Am 1988;70: 122–5. 15. Shirazi-Adl SA, Ahmed AM, Shrivastava SC. Mechanical response of a lumbar motion segment in axial torque alone and combined with compression. Spine 1986;11:914–27. 16. Sung HW, Kuo DP, Shu WP, et al. Giant cell tumor of bone: analysis of two hundred and eight cases in Chinese patients. J Bone Joint Surg Am 1982;64:755–61. 17. Tomita K, Tsuchiya H. Total sacrectomy and reconstruction for huge sacral tumors. Spine 1990;15:1223–7. 18. Tomita K, Kawahara N, Baba H, et al. Total en bloc spondylectomy for solitary spinal metastases. Int Orthop 1994;18:291–8. 19. Tomita K, Kawahara N, Mizuno K, et al. Total en bloc spondylectomy for primary malignant vertebral tumors. XVI Int Cancer Congr 1994;2409–13. 20. Tomita K, Toribatake Y, Kawahara N, et al. Total en bloc spondylectomy and circumspinal decompression for solitary spinal metastasis. Paraplegia 1994;32:36–46. 21. Tomita K, Kawahara N. The threadwire saw: a new device for cutting bone. J Bone Joint Surg Am 1996;78:1915–7. 22. Tomita K, Kawahara N, Hata M, et al. Indication and surgical technique of sacral amputation. OS NOW 1996;22:188–97 (in Japanese). 23. Tomita K, Kawahara N, Baba H, et al. Total en bloc spondylectomy; a new surgical technique for primary malignant vertebral tumors. Spine 1997;22:324–33.