Dec 7, 2002 - 5 mm) consisted of surgical steel (E=200 GPa), the implant material was either titanium (Ti6Al4V; E=110. GPa) or PEEK (or allograft; E=10 GPa; ...
ICBME 2002 Conference, Singapore, 4-7 Dec 2002 BIOMECHANICS OF THE SPINAL MOTOR SEGMENT AFTER FUSION WITH A NOVEL SPACER FOR THE MINIMALLY INVASIVE EXTRAFORAMINAL APPROACH F. K. Fuss*, R. J. Sabitzer+, E. C. Teo*, and K. K. Lee* *School of Mechanical and Production Engineering, Nanyang Technological University (NTU), Singapore, + Orthopaedic Department, Otto Wagner Hospital, Vienna, Austria Introduction The current state of the art for treatment of chronic and painful lumbar pathologies, including degenerative disc disease, pseudolisthesis with foraminal stenosis and postnucleotomy syndrome, is lumbar interbody fusion (LIF). It is primarily intended to decrease pain by decompressing stenotic segments and eliminating motion as well as restoring the segmental lordosis and reduction of stenotic or degenerative spondylolisthesis. LIF is carried out via different approaches: anterior (ALIF), posterior (PLIF), or transforaminal (TLIF). In PLIF, laminas and spinous process are removed surgically (hemi/laminectomy), whereas for TLIF, the facet joint is removed (facectomy). All three approaches share disadvantages and risks, namely a loss of stability, and an extended rehabilitation period. Recently, a new approach was developed in order to overcome these disadvantages: the minimally invasive approach via an extraforaminal approach (ELIF), which is an extraspinal [1,2] usually used for arthroscopic microdiscectomy without fusion implant [3]. The spacer [4,5] used in this study is designed for the ELIF approach. The implant (MicSpace, Aesculap, Germany), currently made of titanium, is small enough (4x1.3cm, area 4cm², height 7-9mm) to fit the extraforaminal compartment. The implant covers the hindpart of the endplate (posterior half to posterior one third of the endplate) and leaves enough space for bonegrafting (Figure (1)) in the anterior part of the intervertebral space. The aim of this study was to analyse the behaviour of the spacer after surgery, and the biomechanics of the implant before and during the fusion process. Methods Seven patients were operated at segment L.3/4 or/and L.4/5 between March and July 2002. The indications ranged from postnucleo /postdiscotomy/ postlaminectomy syndrome and vertebral stenosis to instabilities like spondylolisthesis. In each patient, the spacer was inserted by the ELIF-approach. The bone graft was harvested from the iliac crest of the patient. The condition of the implant after surgery (migration, progression of fusion) was determined radiologically with digital image analysis. The stress distribution of the spacer under compressive loading was assessed using finite element method. The solid model was generated with CAD software, AutoCAD2000, the FE-analysis was carried out with commercially available visualNastran4D v.6.3. The model consisted of 75,000 solid elements. The von Mises stress distribution was calculated at a compressive force of 3200 N, corresponding to a displacement of 0.001 mm. Biomechanical analysis 1: The stress in the implant, in the dorsal stabilisation (rods connecting the pedicle screws), and in the bone graft or fusion bone (BG-FB) was calculated at a compressive load of 3000 N. The application point of the force was in the centre of the
posterior third of the endplates. It was assumed that external and muscle moments are in equilibrium. The plane for the stress analysis was situated in the implant and BG-FB, near to the endplate. The two rods (diameter 5 mm) consisted of surgical steel (E=200 GPa), the implant material was either titanium (Ti6Al4V; E=110 GPa) or PEEK (or allograft; E=10 GPa; Young’s moduli according to [6]). The modulus of the BG-FB was simulated with increasing values from 0-10 GPa. The areas (A), centroids (yC), and 2nd moments of area (I) of implant and BG-FB were calculated with GEO1 v.3.1 (Hexagon, Germany), and reduced to the equivalent steel section according to the modular ratios. Modular ratio of steel: nS = ES/ES = 1 Modular ratio of titanium: nT = ET/ES = 0.55 Modular ratio of PEEK (or allograft): nP = EP/ES = 0.05 Modular ratio of BG-FB: nB = EB/ES = 0 - 0.05 Centroid and neutral axis of the equivalent steel section (the modular ratio of the implant is either nT or nP): yC = (Arod*yrod + nT*Aimplant*yimplant + nB*ABG-FB*yBG-FB) / (Arod + nT * Aimplant + nB * ABG-FB) Equivalent steel section area: A = Arod + nT * Aimplant + nB * ABG-FB 2nd moment of equivalent steel section area: I = Irod + Arod*(yC-yrod)² + nT*Iimplant + nT*Aimplant*(yCyimplant)² + nB*Iimplant + nB*ABG-FB*(yC-yBG-FB)² Moment (M) of the compressive force about the neutral axis (yimplant corresponds to the application point of the force F = 3000 N): M = F*(yimplant-yC) Combined bending and direct stress of the different structures (n and A according to the material and structure; ym = distance between centroid of the equivalent steel section and the anterior or posterior borders of the structures involved): σ = n*(M*ym/I6F/A) This calculation served to simulate the fusion process with increasing stiffness of the BG-FB, and to assess the influences of the two different implant materials used for the fusion. The positions of the spacer (Ti and PEEK, or allograft) were changed from the most posterior position (c.f., Fig. (1)) to the most anterior one, without BG-FB, i.e., immediately after surgery. This served to determine how the position of the spacer influences the stress distribution of BG-FB. Biomechanical analysis 2 (finite element analysis, FEA; ANSYS v.6.0): Using the FE model of Teo and Lee [7], a compressive force of 3000 N was applied uniformly on top surface of L2 vertebra with fully constrained condition on bottom surface of L3 vertebra. The disc was replaced by the spacer and BG-FB (materials according to analysis 1). To account for dorsal instrumentation (pedicle screws and rods) in stabilising the motion segment, a resisting moment was simulated by applying moment on the surface. A parametric script was created to allow the
ICBME 2002 Conference, Singapore, 4-7 Dec 2002 moment applied to increment from a pre-determined value accordingly for each load-step till the FE model has not flexed/extended more than 0.1 degree. This was to simulate the contribution in stabilising the motion segment from the posterior instrumentation. The moment and compressive stress of the BG-FB were then extracted from the analysis. Results Translation and rotation of the spacer after surgery are below 1mm and 1°, respectively. The radiological record shows that the fusion process starts between 6 weeks and 3 months after surgery. The implant’s peak stress is located at the opening of the instrumentation channels as shown in Figure (2). The maximal stress at a compression of 3200 N is 47 MPa. The failure stress of titanium at about 1GPa is reached at an axial compression of 64kN. The results of the 1st biomechanical analysis are displayed in Figure (3). The stress in the BG/FB is about 3 times higher in the PEEK (or allograft) implant compared to the titanium version. The stress in the most anterior position of the implant is 30% smaller than in the most posterior position. The results of the 2nd biomechanical analysis (FEA) show that the stress between endplate and BG-FB is about 2.8 times higher when using the PEEK (or allograft) spacer compared to the titanium implant.
Figure 1: method of insertion (ELIF), and radiograph.
Figure 2: von Mises stress distribution at a compression of 3200 N. Stress scale: white = 0 MPa, black = 47 MPa.
Discussion The most posterior position of the implant, even subdued to the highest stress, is preferable, as it is easily surgically feasible and provides a maximal fusion area. Yet, it is of surgical importance, that any other position of the implant does not offer a disadvantage. Both biomechanical analyses show that an implant with a low Young’s modulus produces a higher stress in the BG-FB entity, and hence influences the fusion process. The stresses in the different materials are far below the failure stress (ultimate yield, tensile, and compressive stress). References [1] Sabitzer, RJ, and Fuss, FK, “The extraforaminal lumbar interbody fusion” 9th Annual Meeting of the Japan Society for the Study of Surgical Technique for Spine and Spinal Nerves (JPSSSTSS), Nagoya; 2002. [2] Sabitzer, RJ, and Fuss, FK, “ELIF: extraforaminal lumbar spinal fusion” 2002 Combined Orthopaedic Meeting Singapore; 2002. [3] Kambin, P, “Arthroscopic microdiscectomy” Arthroscopy; 1992; 8; p 287-295. [4] Fuss, FK, and Sabitzer, RJ, “Implant” PCT-Patent WO99/37255A1; 1999. [5] Fuss, FK, Sabitzer, RJ, and Eckhof, S, “Intervertebral Implant” PCT-Patent WO00/44317A1, 2000. [6] MatWeb (www.matweb.com) [7] Teo, EC, and Lee, KK, “An accurately represented finite element model of lumbar motion segment (L2L3)” Proceedings, Int’l Conference on Biomedical Engineering, Kuala Lumpur; 2002, p 161-163.
Figure 3 a & b: Young’s modulus of BG-FB vs. compressive stress in the structures involved, maximal anterior (bold lines) and posterior (dotted lines) stress; a: titanium spacer, b: PEEK or allograft spacer.
