We examined the radiographs from a prospective clinical study of fixation by pedicle screws and those from an experimental study in a sheep model. In the clinical study, instruments were removed from 21 patients after implantation for 11 to 16 months and the extraction torques of the screws were recorded. A structured protocol was used for the radiological examinations. In the experimental study, loaded pedicle screw instrumentations were implanted in the sheep for six or 12 weeks. After radiological examination the pull-out resistance and the histological characteristics were studied. In the clinical study, all screws with radiolucent zones had a significantly reduced mean extraction torque compared with screws without radiolucent zones (16 ± 10 Ncm Our findings showed that all screws with radiolucent zones had low extraction torques or low pull-out resistance. A radiolucent zone is a good indicator of loosening of a pedicle screw.
Pedicle screws give the best bone purchase of all posterior fixation techniques of the cervical spine, which would suggest a frequent utilisation. However, the cervical pedicles are small and the potential danger of misplacing a screw limits their use. In in vitrostudies the misplacement frequency has been shown to be unacceptably high, whereas this is not seen clinically, maybe due to different insertion techniques. Fortunately a misplaced screw rarely leads to a clinical complication. To minimise the risks, however, we now only use pedicle screws in the cervical spine where stability is critical, i.e. at the extremes of a fixation. For example: A C1–C2 fixation in rheumatoid arthritis or in fracture of the dens would utilise C2–C1 transarticular screws (i.e. C2 pedicle screws). A cranio-cervical or cranio-thoracic fixation would involve 1 or 2 levels of pedicle screws as distal anchorage, and lateral mass screws in between. A short cervical fixation with pedicle screws could be in a trauma patient where it would be desired to have a very reliable fixation with a minimum number of fixation levels. Computer navigation is a promising technique, however, not free from misplaced screws. So far we have experience of 83 navigated screws in 18 patients evaluated with postoperative computed tomography (CT). 67 screws were in correct position, 11 had insignificant breach fractures of the pedicle wall, whereas 4 were incorrectly placed, usually laterally into the foramen for the vertebral artery, none however with a clinical consequence. The main problem with computer navigation in the c-spine seems to be to obtain a good enough CT scan to allow good matching between the virtual and real worlds.
We investigated the effects of hydroxyapatite (HA) coating on the purchase of pedicle screws. A total of 23 consecutive patients undergoing lumbar fusion was randomly assigned to one of three treatment groups. The first received uncoated stainless-steel screws, the second screws which were partly coated with HA, and the third screws which were fully coated. The insertion torque was recorded. After 11 to 16 months, 21 screws had been extracted. The extraction torque was recorded. Radiographs were taken to assess fusion and to detect loosening of the screws. At removal, the extraction torques exceeded the upper limit of the torque wrench (600 Ncm) for many HA-coated screws. The calculated mean extraction torque was 29 ± 36 Ncm for the uncoated group, 447 ± 114 Ncm for the partly-coated group and 574 ± 52 Ncm for the fully-coated group. There were significant differences between all three groups (p <
0.001). There were more radiolucent zones surrounding the uncoated screws than the HA-coated screws (p <
0.001). HA coating of pedicle screws resulted in improved fixation with reduced risk of loosening of the screws.
In a prospective randomised study, the Hansson pin technique for internal fixation of fractures of the femoral neck was compared with the Uppsala screw technique. The series consisted of 115 consecutive patients, 97 women and 18 men, with a mean age of 80 years. There were 56 patients treated with Hansson pins and 59 with Uppsala screws. After one year 22 patients had died, eight in the Hansson pin group and 14 in the Uppsala screw group (p = 0.28). Twenty-four complications had occurred, 18 in the Hansson pin group and six in the Uppsala screw group (p = 0.008). After exclusion of those with complications, the patients in the Hansson pin group had significantly more pain, less mobility, and a smaller proportion were living in their own homes.
We measured the stability of fixation in femoral neck fractures treated with von Bahr screws, investigated the influence of impaction and correlated peroperative stability with the clinical results. Stability was measured at operation using a metal probe fitted with strain gauges. Its tip was anchored in the subchondral bone of the femoral head and its lateral end was fixed in the lateral femoral cortex. The shearing force produced by longitudinal compression applied to the foot of the operated leg was recorded. The results in 41 consecutive patients all followed for 30 months, showed that fractures with early loosening or nonunion had all had significantly poorer stability than the fractures that had healed. Impaction improved stability in only 23 out of the 41 fractures; in the others stability had deteriorated or was unchanged.
We describe a method of internal fixation for femoral neck fractures which has been newly developed to reduce the frequency of early complications. Two cannulated screws are inserted in the axis of the femoral neck to reach into the subchondral bone of the femoral head. The screws are inserted over guide pins and the tip of the screw is self-tapping and designed to provide good anchorage in the femoral head. We used this method in 44 consecutive patients in a prospective study with no exclusions, followed for a minimum of 24 months. All fractures healed within 12 months, and there were no cases of early loosening or nonunion. In four cases, late segmental collapse had developed during the mean follow-up period of 30 months.
