Biomechanics: Lumbosacral Fusion
Lumbosacral Fusion: Cages, Dowels, Pedicle Screws: Part 2
The greatest strength of the vertebral body is present in the subchondral bone of the cortical endplate. The maximal endplate strength is peripheral near the ring apophysis. Two techniques of endplate preparation during interbody fusion are practiced. One involves purposeful endplate cavitation to provide an optimal bleeding bed of cancellous bone. This chapter deals with this technique whereby two cylindrical grafts are screwed into adjacent circular holes oriented parasagitally across a disc space prepared by a reamer which partially removes the subchondral bone and at the apex of the cavity, exposes weak but very vascular cancellous bone. The outer portion of the perforated, cylindrical cage has a continuous threadform that engages the adjacent vertebral bodies and endplates. This threaded design permits insertion by screwing the device into the disc space, which provides resistance to device migration and stabilization to the vertebral bodies, which facilitates spinal fusion. The intervertebral disc space is pre-drilled such that a hole is created which spans the entire height of the disc and includes semi-circular concavities in the vertebral bodies above and below the disc space. The cages, packed with autogenous bone graft, screw into the pre-drilled holes.
The second technique of endplate preparation involves preservation of the subchondral bone. The advantage of preserving much of the endplate and filling the disc space with a greater quantity of bone graft should reduce the risk of graft collapse and increase the fusion rate. This leaves the strongest bone adjacent to the implanted graft and requires a precisely cut graft to exactly match the interspace. The disadvantage, however, is that the endplate is minimally vascularized and the recipient bed is less vascular. Technically, the dowel technique is easier to perform and consistently allows accurate fitting of the cage to the prepared graft bed. By reaming a cavity, the recipient bed is reliably created for the cylinders. The disadvantage with this technique is that the strong trabeculae adjacent to the endplates are breached, increasing the risk of graft settling. The second technique is much more difficult because a perfect fit between host and graft is mandatory and the graft must be perfectly cut to match the subchondral bone surfaces. The accurate insertion of individual blocks is less reproducible than with the dowel technique.
During daily activity, the lumbar spine is exposed to significant biomechanical forces. Studies indicate that a motion segment may experience axial compressive loads ranging from 400 N during quiet standing to more than 7000 N during heavy lifting. 70, 99 The ultimate compressive strength of a non-osteoporotic vertebral body has been reported to be slightly over 10,000 N. 26 Corticocancellous autografts demonstrate inadequate initial mechanical strength for lumbar interbody loading, often leading to collapse or extrusion. 24, 74, 83 The compressive forces across the grafted interspace should be less than that required to induce failure of the graft construct. The graft should be able to transmit force without significant motion so that immediate mechanical load transfer is achieved, and the technique should induce arthrodesis as quickly as possible with minimal to no morbidity associated with its use. Threaded titanium alloy (Ti-6Al-4V) interbody fusion cages have undergone extensive in vitro and in vivo biomechanical testing, demonstrating rigidity sufficient to withstand lumbar spinal loading forces without fracture or deformation. 51, 60, 81, 92 Long-term clinical studies have reported no cases of structural cage failures 60, 83 and cages have been shown to impart increased stiffness as compared to the intact spine. 8, 34, 35, 92 One study comparing biomechanical stability performance among three interbody devices (Threaded Bone Dowels, BAK, and RTFC) found no significant difference under physiological loading conditions. 40 Bone dowels performed as well as titanium cages. In flexion, bone dowels increased stiffness by 970%, Ray increased by 253% and BAK increased by 96%. Under extension, Bone dowels increased local stiffness by 166%, BAK 71%, and Ray 56%. In torsion, bone dowels increased intact stiffness by 20%; BAK also increased global intact stiffness by 20%, while Ray decreased intact stiffness by 5%. Bone dowels increased intact stiffness by 91% under lateral bending, while Ray cages and BAK cages maintained the intact stiffness under lateral bending. None of the implants fractured during failure tests. The vertebral endplate and the sacroiliac joint were found to be the most common failure sites. All devices withstood load to failure, with the vertebral body end plate failing before the implant.40 Threaded cortical bone dowels (Sofamor Danek MD II and MD III) provide an increase in construct stiffness of 68-334% over the intact motion segment. 91 Additionally, the threaded cortical bone dowels demonstrated static compressive strengths of over 24,000 N, well above maximal physiologic loads.
Several biomechanical studies have shown that these threaded anterior interbody devices improve overall stiffness, but are least rigid in extension and axial rotation.63, 75, 87, 92 Oxland et al75. compared the stability of a traditionally paired anterior implantation with that of a lateral implantation technique (preserving the ALL and anterior annulus). The purpose was to test whether this decreased extension rigidity was due to resection of the anterior longitudinal ligament (ALL) and anterior annulus during cage insertion. They found no significant improvement in extension stability with lateral insertion, leading them to conclude this lack of rigidity was associated with distraction of the facet joints after interbody cage placement. Additional posterior instrumentation can provide the added stability required in extension and axial rotation. Supplemental translaminar facet screws that can be placed in a minimal invasive fashion significantly reduce the motion of a BAK biomechanical model in extension and axial rotation. Rathonyi and associates82 found that using translaminar screw fixation can substantially stabilize the problematic loading directions of extension and axial rotation. Volkman94 also demonstrated in a cadaveric biomechanical model that motion segment stiffness of an anteriorly placed threaded spine cage was increased, especially in extension, with transfacet screws.
The ideal interbody graft combines a strong mechanical construct to withstand compressive loads across the disc space while providing an osteogenic, osteoinductive, and osteoconductive matrix. The gold standard for this matrix is autogenous cancellous bone. The compressive strength of this bone, however, is very poor. Combining this with a strong titanium or cortical allograft shell (cage) is sensible. The cancellous autograft iliac crest bone is packed into the cylindrical screw-in cages with the goal that the cage provides mechanical strength to prevent collapse, subsidence, shear, and torsional forces. This produces an optimum stable environment while the autogenous graft grows through the cages into the vertebral bodies above and below. The mechanical strength of the cage is combined with the biologic strength of the autograft. This graft, however, is not biomechanically loaded while it is inside the cage, and the surface area available for the graft to grow through the cage is not large and varies between cage types. Optimally, graft should be packed around and between the cages to maximize the surface area of bone available for fusion and to allow bone graft to undergo physiologic loading. Maximally packing the interspace with bone graft also ensures removal of all disc material and cartilaginous endplate that is avascular and inhibits fusion. With this concept in mind, performing a subtotal "channel discectomy" (only removing a cylindrical channel of disk material using a drill) that occurs in the laparoscopic technique, is not optimal. 66 This partial "reamed channel" discectomy results in a limited fusion confined to a small cross-sectional area (the fenestrations in the cage). In a prospective, randomized study66 of BAK cages packed with autogenous iliac bone graft, a complete discectomy vs. partial reamed channel discectomy was performed in 100 patients. All 50 patients in the complete discectomy group achieved a solid arthrodesis at a mean follow up 25 months with no revision surgical procedures. In contrast, 7 patients in the partial reamed channel discectomy group had a pseudarthrosis with 8 patients required revision surgery. The difference between the groups was significant (p=0.019).
One conceptual problem associated with cylindrical interbody fusion devices, both titanium cages and threaded cortical bone dowels, is their geometric shape. The volume available for bone graft in cylinders is less than that in vertical ring devices, such as the femoral ring allograft.98 A tapered device, as opposed to a cylindrical shape, which has identical anterior and posterior height, 92 better restores lordosis and sagittal balance. The segmental lordosis and wedge shaped anatomy present in the human intervertebral disc space results in non-uniform implant contact, anterior to posterior.4 Additionally, it has been calculated that the BAK cage allowed a maximum interface with only 10% of the total surface area of the end plate.98 Some authors have concluded that the interbody bone graft area should be significantly greater than 30% of the total end plate area to prevent failure.16 However, to increase interbody graft contact with high quality bony bed, greater amounts of subchondral bone need to be removed, increasing the risk of subsidence.87 In a sheep in-vivo model, a threaded titanium interbody fusion device was compared to an anterior fusion using autogenous iliac crest dowel graft. After surgery, interbody distraction successfully occurred in cage and autograft sites. Loss of interbody height ensued in both groups during the first 2 months. Percentage loss of height was lowest in the cage sites. Both techniques effectively distracted the intervertebral spaces beyond their baseline measures. The cylindrical cages nearly doubled the normal vertical span of the disc spaces. All, however, experienced subsidence of disc height during the first 2 months. Although the absolute reduction in intervertebral height was similar between the groups, the cage sites lost a smaller fraction of their initial distraction. At final measure, the cage-implanted sites had lost 19.6% of their postoperative height but remained well above the normal disc height.87
Unlike titanium interbody cages, threaded cortical bone dowels are subject to supply shortages and processing problems. Presently, the majority of threaded cortical allograft bone dowels are obtained through aseptic harvest techniques with subsequent processing steps occurring in "class 10 certified" clean rooms. After appropriate donor screening tests and chemical processing with hydrogen peroxide and 70% ethanol, the bone dowels are freeze-dried or frozen. This often avoids the necessity of terminal sterilization by high-dose gamma irradiation or ethylene oxide- methods that can impair the mechanical and physiologic properties of the allograft.27 The biomechanical properties of allograft bone can be altered by the methods chosen for its preservation and storage. These effects are minimal with deep-freezing or low-level radiation. Freeze-drying, however, markedly diminishes the torsional and bending strength of bone allografts but does not deleteriously affect the compressive or tensile strength. Irradiation of bone with more than 3.0 megarad or irradiation combined with freeze-drying appears to cause a significant reduction in breaking strength.77
There have been no documented cases of HIV transmission from musculoskeletal allografts since 1985, although there have been over 7 million bone and soft tissue transplants performed since that time. Utilizing current-generation PCR screening tools, the risk of HIV transmission is estimated to be approximately one in eight million.9