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Fifteen fresh-frozen adult cadaver feet were dissected to investigate areas in the hindfoot where external fixation pins could be safely inserted with the least risk to underlying nerves, vessels, and tendons. Using palpable anatomic landmarks, four relative "safe zones" on the calcaneus and talus were delineated. These included an area on the medial calcaneus, the medial talus, the lateral calcaneus, and the lateral talus. The medial calcaneal safe zone was a large, easily definable rectangular area on the posterior aspect of the tuberosity, posterior to the neurovascular bundle and extrinsic tendons. The medial talar safe zone was located on the medial talar neck, anterior and superior to the tibialis posterior tendon. The lateral calcaneal safe zone consisted of a large area of the lateral calcaneal tuberosity, located posterior to the peroneal tendons and sural nerve trunk. The lateral talar safe zone included only a narrow, vaguely palpable, quadrangular area on the lateral neck of the talus. The medial safe zones could be easily delineated by palpation and appeared safe for routine unilateral external fixation across the medial hindfoot and ankle. The lateral safe zones appeared safe and useful if both medial and lateral frames were required. The structures most at risk for injury during pin insertion in the zones described were the medial and lateral calcaneal nerve branches, which inconsistently crossed the medial and lateral calcaneal safe zones, respectively. In these areas overlying the tuberosity, however, the subcutaneous tissues were thin, and iatrogenic nerve injury during pin insertion appeared avoidable if blunt dissection was used to reach the calcaneal cortex. The data presented here provide information to assist selection of pin sites that minimize risk to underlying soft tissues during external fixation of the talus and calcaneus.

A retrospective review of patients treated with internal fixation of fractures or dislocations of the hand or wrist over a four-year period was undertaken to determine the complication rates of pin fixation for stabilization of these injuries. One hundred thirty-seven patients who received 422 pins were studied. All pins were unthreaded, measured 0.035-0.069 inches (0.9-1.8 mm) in diameter, were placed with a power drill, and were left protruding through the skin. The mean time that pins were left in place was 6.5 weeks, (range, two days to 24 weeks). Minimum follow-up time was 43 days after pin removal. Thirty-four complications occurred in 24 patients, and the overall complication rate was 18%. Forty-five of the 422 pins were involved (11%). Complications included infections in ten patients (7%), pin loosening without infection in six (4%), loss of reduction in six (4%), symptomatic nonunion in six (4%), impaled flexor tendon in two (2%), asymptomatic pseudarthrosis in one (1%), pin migration in one (1%), median nerve injury in one (1%), and radial artery injury in one (1%). Osteomyelitis developed in two of the patients with infections. Pin tract infection occurred at a mean time of ten weeks and aseptic loosening at a mean time of eight weeks. The frequent complication rates emphasize the need for meticulous pin placement, adequate intraoperative evaluation of pin position, and satisfactory patient compliance. Despite the frequency of these complications, serious permanent sequelae did not occur in most patients.

Osteomyelitis and intracranial abscess are among the most serious complications that have been reported in association with the use of the halo device. The cases of five patients who had formation of an intracranial abscess related to the use of a halo cervical immobilizer are described. All of the infections resolved after drainage of the abscess, débridement, and parenteral administration of antibiotics. Meticulous care of the pin sites is essential to avoid this serious complication. Additionally, since all of the infections were associated with prolonged halo-skeletal traction, this technique should be used with caution and with an awareness of the possible increased risks of pin-site infection and of formation of a subdural abscess.

It has been recommended that halo torque screwdrivers be used only once. To determine if this advice is justified on biomechanical grounds, five commercially available torque screwdrivers were cycled 1,600 times each in an Instron testing machine over a 7-day period. The screwdrivers displayed an initial error from the dial reading at both 6 and 10 in-lb of only +/- 0.3 in-lb. After cyclic loading and retesting, the percent deviation from original calibration was 8% at both 6 and 10 in-lb. These data demonstrate the accuracy and dependability of the torque screwdrivers and imply that repeated use of these devices should be safe.

Twenty-seven cadaver skulls and 20 CT scans of skulls were measured above the orbital rim and ear but below the greatest diameter to determine optimal placement of halo pin sites. At the antero- and posterolateral portions of the calvaria, the outer table averaged 2 mm, the diploe 3 mm, and the inner table 2 mm. At the temporal fossa, the outer cortex averaged 1.7 mm, the diploe 2.0 mm, and the inner table 1.6 mm. Average distance between the anterior edge of the temporal fossa and the frontal sinus approximated 3 cm. These data confirm previously recommended halo pin insertion sites, anterolaterally and posterolaterally, where the bone is thickest and the thinner frontal sinus and temporal fossae are avoided.

The rates for loosening and infection of the pins used in the halo apparatus are unfortunately high. The commonly recommended amount of torque to be used in applying the pins is 0.68 newton-meter (six inch-pounds). Forty-two adult patients underwent application of a halo device for immobilization of the cervical spine using an increased torque of 0.90 newton-meter (eight inch-pounds). The rate for loosening of the pins and the rate for infection at the pin site dropped from 36 per cent to 7 per cent and 20 per cent to 2 per cent, respectively.

The structural response of the halo orthosis pin-bone interface to transverse loading was evaluated on an Instron testing machine using fresh cadaver calvarium sections. Commercially available stainless steel (control) pins and newly designed stainless steel experimental pins were evaluated. Cyclic loading tests and load-to-failure tests were performed. Of the many designs tested, one pin demonstrated an improvement in structural properties at the pin-bone interface compared with the control pin. Furthermore, the new pin design was more resistant to insertional torque reduction when subjected to cyclic loading after insertions at 4 and 6 in-lb. Both the control and experimental pins exhibited improved structural behavior at 8 in-lb of insertional torque compared to the currently recommended 6 in-lb.

Skull bone thicknesses of 18 pediatric skulls were measured by means of computerized tomography scans to determine optimal sites for placement of halo pins. Skulls were grouped to assess changes with age: Group 1, 1-2 years; Group 2, 2-5 years; and Group 3, 5-12 years. At the antero- and posterolateral portions of the calvaria, the average total thickness was 3.7 and 3.9 mm, respectively, in Group 1; this value increased relatively linearly to 6.1 and 5.9 mm, respectively, in Group 3. At the temporal fossa, the average thickness increased from 3.0 mm in infants to 4.1 mm in preteenagers. These findings confirm clinical recommendations that halo fixation pins should be placed anterolaterally and posterolaterally where the bones are thickest.

A halo skeletal fixator was studied prospectively in 51 patients using two different pin insertion techniques. One technique consisted of a stab incision prior to pin placement and the second technique used no incision. There was no difference in infection, loosening, or resultant scars between the two groups. Incision for halo pins may increase initial bleeding and seems unnecessary.