Fig. 17.1
Cervical Cage “Duosorb” ™ (SBM, Lourdes, France)
Fourty patients, suffering of cervical nevralgia in relation with soft disc herniation, were operated according to the Smith and Robinson technique [1], Clinical evaluation at pre-op, post-op and each follow-up visit included JOA scoring, neurological evaluation, AP and lateral Xrays with bending films. At 2 years follow-up a CT scan evaluation of the fusion was performed to evaluate the fusion quality and the resorption of the cage and X rays at 4 and 6 years.
Preoperatively templates were used to determine the right size of the implant, and the cages were softly introduced in the disc space. Further insertion was carefully performed with a polyethylene bone impactor. Doing so, the implants were located at the center of the disc, ensuring vertebral end-plates parallelism, which was confirmed via peri-operative radiographic controls. All the patients were stabilized by an anterior titanium plate fixed by four screws and locking mechanism (Zephyr plate, Medtronic, Memphis, USA).
Verticalization was allowed the day after surgery, with no particular care except a soft collar, further maintained during 10 days. A 24 h post-operative radiographic control was performed. Data were collected from a homogenous consecutive series of patients, operated between 2004 and 2005. The same surgeon assumed reviews 1, 3 and 6, 12 months and each year after surgery, then at latest follow-up.
For each of the patients, we have postoperatively evaluated the clinical benefits of surgery and possible sequels : dysphagia, radicular or cervical residual pain. Correction (disc height and kyphotic/lordotic deformation), displacement and/or fracture of the implant, radiolucent line, dynamic mobility of the vertebrae (flexion/extension X-Rays) were radiologically assessed at each examination. The resorption of the material was evaluated radiographically by an independent radiologist in four stade: no resorption, partial resorption (inferior to 50 %); major resorption (more than 50 %); complete resorption. Finally, twenty patients got a CT evaluation of the fusion at 2 years follow up.
Results
Twenty patients included in the study were all followed at 2 years, mean age 48 years, average follow-up 28 months (Min 18/Max 47). The main syndrome at first examination was cervical pain (30 %), radicular syndrome (60 %), myelopathic syndrome (15 %). A single level fusion was performed in all the patients.
From a clinical point of view, we observed one dysphagia. Improvement in initial pain syndrome was noted for 80 % of the patients. Radicular pain syndrome is present at latest follow-up for 15 % of the patients, as residual cervical syndrome is noted for 40 % of the patients. Complete disappearance of pain occurred for 80 % of the patients who initially presented radicular syndrome and for 50 % of the patients who presented preoperative cervical pain.
Radiological findings evidenced that 95 % of the patients conserved initial correction, with neither measurable (<1 mm) loss of disc height nor kyphotic lordotic evolution of the curve. One patient showed impaction of the implant inside the lower vertebra with a 3 mm loss of disc height. In 100 % of our observations, no measurable (<1 mm) implant displacement was observed at latest follow-up. Flexion/extension X-ray showed no mobility of the grafted level(s) for all the patients, for whom fusion was considered to be acquired. A moderate radiolucent dark line was observed in two of our observation in contact with one vertebral end plates. Its thickness decreased over time, and becomes invisible at 6 months and at latest follow-up. No visible signs of implant resorption was noticed using routine x rays at 6 months follow up.
No cyst or lysis where detected on CT scan analyzis. The quantity of new bone formation at the cage level was evaluated as 62.6 % (12.4 sd) of the volume of the cage at 2 years. No infection, no neurological complications were reported.
Discussion
In animal studies histological examinations indicated clearly that pure PLLA implants were systematically surrounded by a layer of conjunctival tissue containing large numbers of macrophages, irrespective of the length of implantation, whereas bone tissue grew in direct contact with composite implants during the first few months and matured gradually until the end of the study period with no marked macrophage activation. The composite material under investigation is thus more favorable for bone consolidation in vivo than pure PLA and has identical properties to those of pure b-TCP ceramics. This result is not particularly surprising if we consider the properties of the materials in the composite separately. We know that pure lactic acid-based polymers are not cytotoxic or inflammatory in vitro [4, 8, 24, 26–30] but are capable of causing an inflammatory reaction in vivo [4, 8, 26, 30–33], as confirmed by clinical observations [10, 18, 19, 34–36]. Pure b-TCP, on the contrary, promotes bone healing by osteoconduction, becoming surrounded by healthy bone tissue containing active osteogenous cells, with no intermediate fibrous layer. It is, thus, conceivable that composite materials such as those we studied have a “hybrid” biological behavior that becomes closer to that of pure tricalcium phosphate as the mineral content increases.
This clinical series showed that fusion occurred within regular time period. No inflammatory reaction was noted confirming in vivo study. Radiolucent line in two cases disappeared quickly which might the sign of progressive bone healing process around the implant. No loss of disc height was noted at latest follow up assessing the good behaviour of the implant when an anterior plating is performed. This preliminary results needs further studies to confirm strength retention properties of the implant and its resorption rate over time. We haven’t observed radiological resorption of the cage during the time of the study despite significant resorption was seen in animal study. This could be attributed to the neutralization effect of the anterior plate decreasing the stresses applied on the cage. Since no loss of correction was observed we can admit that the mechanical properties of the cage was maintained during this period otherwise screw mobilization would probably have appeared.
Conclusion
This new composite material eliminates the inflammation reaction induced by the use of PLA alone and promotes new bone formation. The ceramic block guarantees the maintenance of the disc height and its slow resorption allows long term fusion and stability. At the end this combination provides the same results as tri-cortical iliac crest graft without the disadvantages of the bone harvesting site. Further clinical studies are requested to confirm these results.
References
1.
Smith GW, Robinson RA. The treatment of certain cervical-spine disorders by anterior removal of the intervertebral disc and interbody fusion. J Bone Joint Surg Am. 1958;40-A:3. Williams DF. Enzimatic hydrolysis of polylactic acid. Eng Med. 1981;10:5–7.
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Fuchs M, Koster G, Krause T, Merten HA, Schmid A. Degradation of and intraosseous reactions to biodegradable poly-L-lactide screws: a study in minipigs. Arch Orthop Trauma Surg. 1998;118(3):140–4.CrossRefPubMed
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