Total lumbar disk prosthesis: biomechanics

Total lumbar disk prosthesis: biomechanics and technique of insertion

J. C. Le Huec, S. Aunoble, Y. Basso, T. Friesem, H. Mathews, T. Zdeblick

The treatment of chronic low back pain due to advanced disk degeneration requires restabilization of the lumbar column, mainly by restoring disk function. Disk restabilization involves restoration and stabilization of the ligaments and annulus. Such restabilization represents a new concept [14, 29] in treating disk pathologies without the definitive loss of disk function [11, 13, 26]. Unlike fusion [20], the procedure is not definitive and makes it possible to restore function and mobility. Two prosthetic devices have now been on the market for about 10 years: the SB Charité [6, 8, 12, 15, 18, 27, 35], produced by Link, and the Prodisc [3, 30], manufactured by Spine Solution. the Maverick disk prosthesis is the last player and has been developed according to biomechanical principles [21] in the light of results obtained with the SB Charité and Prodisc.

The aim of this article is to review the advantages and disadvantages of each of the 3 main prosthesis and at the end to try to find if there are specific indication s according to the biomechanics. Many characteristics must be analyzed.

Mobility and stiffness

Re-establishing disk mobility is one of the aims when using a disk prosthesis. Many authors have demonstrated the mobility of such devices 10 years after insertion, including Lemaire et al. [27] and Griffith et al. [15] with the SB Charité prosthesis, and Bertagnoli and Kumar [3] with the Prodisc. The mobility obtained is about 6° in flexion and extension at the L5-S1 level and 8° at L4-L5 [27]. Mobility in lateral inclination has also been analyzed, found to be about 3°to 4°. Lemaire et al. [27] analyzed rotational mobility with the SB Charité by investigating torsion on normal cadaver spines compared to others fitted with the SB Charité. The results were very interesting, because they demonstrated that a spine fitted with the SB Charité had a degree of mobility in torsion that was more than 140% that found in normal spines [27]. Rotational hypermobility may therefore be obtained. Such a finding has not yet been reported with the Prodisc. The Maverick device has also been tested with the same torsion mobility protocol, and compared with a healthy spine [28]. The results were slightly inferior to those obtained in a normal spine, i.e., a certain stiffness was found in segments fitted with the device (Fig. 4)[CE1] . These findings are important to bear in mind because, although the aim of restoring disk function is to obtain a degree of flexion, extension and lateral inclination close to normal values, it is not desirable to end up with a degree of torsion greater than normal. Such hypermobility might induce an overload on the posterior facets, risking early degeneration. Moreover, the posterior structures of patients treated in this way are not fully intact [5, 7], owing to the disk degeneration, so any hypersollicitation of the posterior facets may induce pain. This point is important to know when the indication for surgery is taken.

Stability

The SB Charité device is composed of three parts: two metallic plates and a polyethylene insert. This biomechanical concept provides scope for five degrees of freedom, but it also involves the risk of dislocation to the polyethylene core. There have been very few reports of such dislocation in the literature [33], but wear to the polyethylene may favor this risk because the device is not intrinsically stable owing to the polyethylene implant. The Prodisc device is also composed of three parts, but the polyethylene insert is fixed to the lower endplate, thereby normally avoiding the risk of dislocation. However, the latter may occur and has been reported by Aunoble et al. [1]. The Maverick device comprises only two metal parts, according to the ball and socket principle. No dislocation is therefore possible.

The anchoring of the device to the vertebral endplates is also a very important feature. When it was first introduced, the SB Charité prosthesis was not solidly anchored, and this led to several cases of early expulsion of the device [6, 33]. Subsequently, the technique was modified to include a hydroxyapatite coating, and the resulting bony growth around the vertebral endplates seems to have greatly reduced this risk. However, isolated cases of early expulsion have still been reported for the SB Charité, particularly by Van Ooij [33]. It would therefore seem that hydroxyapatite coating around the endplates is not in itself sufficient to stabilize the implant.. The Prodisc, in contrast, has had not a single reported case of expulsion due to an anchoring defect over the now 10 years it has been in use [3, 30], which would seem to indicate that the system of anchoring by fins, used by the Prodisc, is much more reliable over time. This is the anchoring system used by the Maverick too

Wear tests

No data regarding polyethylene wear in the SB Charité and Prodisc devices have yet been published. Both devices use a combination of polyethylene and metal, and the only comparative data available are those concerning polyethylene-metal devices for hip replacement [32]. According to Hedman et al. [17], the lumbosacral spine undergoes about 125,000 significant flexions per year. A significant flexion can be considered to correspond to the forward flexion of the spine when raising a 20-kg load. On the basis of this finding, it is possible to test disk prostheses. It may therefore be considered that 10 million cycles of flexion, extension, lateral inclination and rotation correspond to clinical use of about 31.5 years [28]. Such tests have been performed with hip prostheses, and it has been reported that metal-polyethylene prostheses produce between 1,354 and 4,500 mm³ of debris [32], and that metal-metal hip prostheses produce about 157.5 mm³[16, 34]. Under the same conditions of use, the Maverick device produced between 12 and 14 mm³ of debris [28], so the amount of debris produced by the metal-polyethylen hip prosthesis was greater than that produced by the Maverick metal-metal disc prosthesis by a factor of 97 and 322 [19, 31]. The risk of the Maverick device wearing out is therefore extremely low.

However, it is important to assess the toxic potential of metallic debris on the surrounding structures [31]. Allen et al. [2] analyzed the toxicity of metallic debris in vitro on cell cultures of osteoblasts at concentrations of 0.01, 0.1 and 1 g/cm³. They showed that at a concentration of 1 g/cm³, the expression of alkaline phosphatase and osteocalcin was inhibited. Since the production of metal debris by the Maverick device is about 0.1 g/cm³ after 61 years of use [28], its toxic potential is ten-fold lower than the toxic threshold on the cells after this period of time. Therefore, the metal-metal combination seems perfectly suited to obtaining the optimal longevity of the device. An epidural toxicity of the wear debris of the Maverick was conducted on rabbits and reported by Le Huec [25]. The results showed that there was no difference between controls and tested animals. In cases of total hip replacement, the polyethylene wear debris is responsible for prosthesis loosening over time, due to the macrophagic reaction induced by the macro particles [32]. This has never been reported with disc prosthesis.

Capacity to absorb shocks

It is essential to understand the shock-absorbing potential of a disk prosthesis. In fact, this sixth degree of freedom has not yet been analyzed for any of the devices discussed so far. Only the Acroflex device [9], a metal-polyethylene prosthesis,[CE2] has been reported to possess this capacity, but it is not commercially available, and, so far, studies have only been performed in animals [9]. Metal-polyethylene devices could therefore have a better capacity to absorb shocks, and this might be an advantage. In order to establish whether this is the case, the Prodisc metal-polyethylene prosthesis and the Maverick metal-metal device were compared in terms of their shock absorption and transmission of vibrations, in a study conducted by Le Huec et al. [23]. The preload applied was 350 N, and a 100-N overload was applied with a frequency varying from 0 to 100 Hz. The data showed that at the sensitivity threshold of the measuring devices used, there was no difference between the implants regarding shock absorption or the transmission of vibrations. The two implants would therefore seem to have similar dynamics, with neither having any detectable shock absorbing effects.

Technique for inserting total lumbar disk prosthesis

The aim of a total lumbar disk prosthesis is to recover the mobility of the segment concerned and its stability. Given the indications [22], such as discopathy with loss of disk height and inflammation of Modic type I or II in vertebral endplates eventually associated with osteophytic involvement, it is essential before inserting the prosthesis to prepare the disk space.

The prosthesis is introduced by an anterior approach [24]. We prefer the video-assisted retroperitoneal approach, which is much less invasive and does not destroy the muscle structures, thus helping to maintain the stability of the lumbar column [24]. The prosthesis is implanted anteriorly, and it is essential to establish a minimum width of 35 mm on the anterior part of the disk to perform the implantation safely. After opening the anterior annulus, which is kept at the end of the intervention[CE3] for protective purposes, the disk is removed as far as the posterior annulus, while the lateral annulus is conserved on both sides. The posterior longitudinal ligament is not sectioned, but a posterior release is performed in order to mobilize the vertebral segment. In fact, owing to the frequent presence of osteophytes, there may be very considerable posterior adhesions and very low posterior mobility of the vertebral segment. It is therefore essential to perform posterior release with ablation of these osteophytes and partial release of the posterior longitudinal ligament on both sides of the vertebrae. This release must be checked during the intervention with intra-operative control during parallel distraction of the vertebral endplates. If this release is not performed, then the disk will have an anterior opening that could lead to a prosthesis being inserted in an excessively oblique position, which would not be physiological. It is also important to remove the fragments of the annulus or osteophytes at the entry to each foramen, in order to avoid pushing them into the foramen during implantation. The vertebral endplates must be prepared, but the subchondral bone must not be removed in order not to weaken the plates, so guarding against any risk of subsidence. Pre-operative bone densitometry in subjects aged over 55 years is therefore essential in order to ensure the bone quality is good. The instruments used must make it possible to determine the angle of the prosthesis, the height to be implanted, and the width and depth of the implant. For this purpose, templates may be used to assess the optimal size of implant to be used on pre-operative computed tomography (CT) scan or magnetic resonance imaging (MR) cuts. Intraoperatively, it is possible to assess the obliquity of the disk after release and its height. The choice can then be made with the instrument provided.

Conclusion

The TDA device is a promising therapeutic technique. Its mechanical characteristics and biomechanical properties make it an interesting option in terms of its life cycle. Only long-term follow-up exceeding 5 or 10 years with prospective analyzis will make it possible to confirm these very favorable preliminary results and to analyze the effects on the segments adjacent to the levels operated.
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Fig. 1. The 3 players: SB Charité, Prodisc, Maverick [CE14]