ESTUDO MULTICÊNTRICO USE OF A NEW EXPANSIVE SCREW IN PATIENTS WITH POOR BONE QUALITY . MULTICENTER STUDY

OBJETIVO: Observar el comportamiento del tornillo expansivo en pacientes con mala calidad osea, su seguridad, tecnica, manejo y complicaciones: porcentaje de aflojamiento, roturas, "pull-out" y pseudartrosis. METODOS: Realizamos estudio prospectivo multicentrico analizando factores de riesgo del paciente, VAS, tiempo quirurgico, perdida de sangre, emplazamiento de tornillos y complicaciones debidas al implante a la alta y a los 3, 12 y 24 meses. RESULTADOS: El 99% de los pacientes no tuvieron ninguna complicacion permanente relacionada con el implante; solo hubo un caso de radiculopatia no resuelta. En el 95% de los implantes, los tornillos se colocaron sin complicaciones; en el 5% aparecieron complicaciones relacionadas con la mala colocacion o expansion del tornillo, resueltas en acto quirurgico. Tiempo quirurgico promedio por nivel, 56 min.; tiempo promedio por intervencion, 2 horas 35 min. Sangramiento promedio por nivel intervenido, 211cc. Hemos tenido tres casos de "pull-out". El VAS evoluciono favorablemente de forma significativa, con reducciones promedio mayores a cuatro puntos. El estudio continuara hasta los cinco anos, siendo estos los resultados preliminares. CONCLUSIONES: Este tipo de tornillos expansivos aportan un nuevo sistema de anclaje para pacientes con mala calidad osea; son seguros y eficaces, ofrecen rapidez en su colocacion, una menor exposicion a los RXy, en caso de retirada del tornillo, dejan el camino libre para una nueva cirugia.


INTRODUCTION
Due to the increased longevity of the population, it is becoming increasingly necessary to perform surgery on elderly patients with spinal pathology, and it is well-known that over the age of 25, bone quality decreases with age, and is considered poor after the age of 65 years.
The spongy bone is the most affected, with a decrease in quantity and changes in the microstructure of the bone.
It has been demonstrated that fixation of implants is less effective when the bone is of poor quality.The same applies to the vertebra, for which biomechanical studies have demonstrated higher rates of screw loosening, pull-out and pseudoarthrosis. 1,2arious methods have been proposed for improving screw fixation in cases of poor bone quality: the use of longer screws with a wider diameter; the use of screws with cementation; and size of the graft inside the vertebral body.In theory, these methods enable greater fixation of the screws.4][5][6][7] Furthermore, biomechanical studies have failed to demonstrate improved fixation of the implants or greater resistance to pull-out with some of these methods. 3,4[10] We present a study with an expandable screw with a new design, which doubles its diameter at the level of the vertebral body, where the pedicle ends, increasing resistance to pull-out by 30% and resistance to mechanical failure by 50% in poor quality bone, compared with the normal screw. 8he objective of this study is to evaluate the reliability of the expansive screw in the treatment of instrumented vertebral fusion in patients with poor quality bone, in terms of its management and safety.

METHODS
We carried out a prospective, multi-center observational study in 99 patients with different spinal pathologies (fractures, deformities, degenerative diseases) in which we placed a total of 576 expandable screws OsseoScrew, Alphatec Spine) with a mean follow-up of 19.2 months (σ=6.6;12-29).(Figures 1 and 2) The main biomechanical characteristic of these titanium screws is their capacity to expand in the union of the middle and distal thirds of the screw body itself, after it has been inserted in the vertebral body immediately anterior to the pedicle.This expansion capacity is up to double the diameter of the screw at the union of the middle and distal thirds of the screw, giving three anchorage points: at the start of the pedicle with the head of the screw, in the pedicle itself, and in the vertebral body at the point where it separates from the pedicle.(Figure 3) We studied the reliability of the fixation technique and its management, through the analysis of surgery time, blood loss, and screw placement; security of the implant, analysis of the complications caused by it; and analysis of the evolution of postoperative VAS as an indirect value of the non-existence of complications.
We performed a study in 99 patients, with an average of 73 years (52-83) and a prevalence of females (67%) compared to males (33%).
Regarding the reason for surgery, the majority of the patients were operated on due to degenerative pathology of the spine (82%), followed by traumatic pathology (11%) and deformity (7%).
The main motive for the intervention was due to lumbalgia or lumbosciatalgia, followed by claudication and/or stenosis of the canal.In a smaller percentage, the patients were operated on for paresthesias of the lower limbs, deformity and/or scoliosis, discopathy and/or chronic radiculopathy, and changes in sagittal profile.
The screws were placed mainly in the last segments of the lumbar spine (L4, L5 and S1).In one case, the screws were placed from T3 to S1, in one patient with adult scoliosis, this being an exception, due to limitation of the diameter of the screw and of the pedicle at levels higher than T10.
In 49% of cases, circumferential arthrodesis was performed (40% somatoartrodesis with implant via PLIF, 35 via TLIF and 6% only with contribution of autograft of the iliac crest).

Surgery time
The average surgery time was 2 hours and 35 minutes, with the majority of cases lasting less than 3 hours (55%, less than 3 hours; 45%, more than 3 hours) and an average time per level of 56 minutes.

Blood loss
The average intraoperative blood loss, analyzed by measuring the volume in cubic centimeters (cc), was 652 cc per intervention,

Module
Low High and between 301 and 1000 cc in most cases (33%).This average was increased in cases of deformities.We recorded an average blood loss per instrumented vertebra of 210 cc.

Intraoperative complications
In most of the patients who underwent surgery, there were no complications derived directly from the screw (95%).There was one case of partial rupture of one of the pedicles of L4; three cases of failure of the screw to expand; one medialization of a screw in S1, which was resolved by retrieving the screw and placing a new one a month after the surgery, and one case in which a malpositioned screw had to be replaced in the same surgical procedure.

Evolution of VAS
Analyzing the evolution of pain between the preoperative and postoperative periods, in the majority of cases we found a reduction in VAS of more than 4 points, both in terms of lumbar pain and irradiated pain.

Follow-up results
In the two years after surgery, there were three cases of pull-out, in two patients.

DISCUSSION
[10][11][12] Some studies on the effect of pedicle screw fixation by bone mineral density (BMD), have shown greater resistance to pull-out and loosening than conventional screws in patients with lower BMD. 1,2bsequently, other authors have shown that in these patients with poor bone quality, the resistance to loosening using expandable screws was similar to that of normal screws in healthy bone. 8,12he resistance to loosening and pull-out has also been shown to be higher with the use of cemented screws in patients with poor quality bone, compared with conventional screws.However, different studies in vitro have proposed the possibility of complications due to leakage of the cement.Furthermore, it has been proposed that in the long term, the occurrence of loosening was similar to that recorded with the use of conventional screws.For this reason, some authors recommend the use of expandable screws with cementation in patients with less severe poor bone quality, reserving the expandable screw and cementation for severe cases. 4,6,7egarding the grade of arthrodesis obtained, different studies have demonstrated a higher percentage with expandable screws, in patients with poor quality bone, than with conventional screws. 11n general, there have been few studies in the literature on expandable screws.The majority compares them with cemented screws, and all are biochemical studies.

CONCLUSIONS
In view of the results obtained in our studied, which corroborate other studies on expandable screws, we believe that the use of this screw in patients with poor quality bone is a secure method, with few complications derived from the implant, and with a low percentage of pull-out and loosening of the screws.We therefore consider it to be a useful method for use in patients with poor quality bone.