title: Evaluation of skull thickness and insertion torque at the halo pin insertion areas in the elderly: a cadaveric study.
abstract: BACKGROUND
  The halo skeletal fixator provides the most rigid type of immobilization of all the orthoses that stabilize the cervical spine. Sometimes with older patients (>70 years old), the pin penetrates the cortical and cancellous bone of the skull and enters the intracranial space, which can result in serious complications such as brain injury, infection, hematoma, and loss of cerebrospinal fluid from the subarachnoid space. Currently, there is a lack of relevant literature that examines these concerns.
  
  
  OBJECTIVE
  To evaluate the thickness of the outer table, diploe, and inner table at the anterolateral and posterolateral pin insertion areas of the skull in elderly cadavers by using computed tomography (CT) scans. In addition, insertion torques at the four standard pin insertion areas was determined by applying halo pins at incremental torque in an effort to suggest safe torque levels for the anterolateral and posterolateral pins.
  
  
  METHODS
  A human cadaveric anatomical and biomechanical study relating to thickness and insertion torques at standard pin areas in the elderly.
  
  
  METHODS
  Twenty-one elderly cadaveric skull specimens.
  
  
  METHODS
  Thickness of cortices (tables) and diploe of skull and insertion torque at halo pin insertion areas.
  
  
  METHODS
  Aquarius Image software at the CT scanner's TeraRecon Aquarius Workstation was used to make the necessary skull thickness measurements at the pin insertion areas. Six, 8, 12, 18, and 36 inch lb of torque were used to determine penetration of the pins through the inner table at each of the four locations (two anterolateral and two posterolateral).
  
  
  RESULTS
  The mean anterolateral thickness was 7.36+/-1.57 mm. The average thickness of the outer table, diploe, and inner table were 2.24+/-0.44 mm, 1.52+/-0.41 mm, and 3.59+/-1.70 mm, respectively. The mean posterolateral thickness was 9.47+/-1.12 mm. The average thickness of the outer table, diploe, and inner table were 4.32+/-0.92 mm, 1.88+/-0.35 mm, and 3.27+/-1.21 mm, respectively. No pin penetration was seen at the traditional 8 inch lb of insertion torque in both the anterolateral and posterolateral pin insertion areas. Eighteen inch lb of torque resulted in penetration in 90.48% (19/21) and in 85.71% (18/21) of specimens in the left anterolateral and right anterolateral pin insertion areas, respectively. No penetration was seen even at 36 inch lb of torque in 80.95% (17/21) of the cadavers in both the left and right posterolateral pin insertion areas.
  
  
  CONCLUSIONS
  The current study supported previous research that 8 inch lb of torque is safe for application of halo pins in the elderly. The posterolateral skull is thicker and stronger than the anterolateral skull. The safe maximum torque is 8 inch lb for anterolateral pin insertion area and 18 inch lb for the posterolateral pin insertion area.
authors.affiliation: Department of Orthopaedic Surgery, Medical University of Ohio, Toledo, OH 43614, USA.
authors.forename: Nabil A
authors.initials: NA
authors.lastname: Ebraheim
authors.forename: Jiayong
authors.initials: J
authors.lastname: Liu
authors.forename: Vishwas
authors.initials: V
authors.lastname: Patil
authors.forename: Chris G
authors.initials: CG
authors.lastname: Sanford
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Crotty
authors.forename: Steven P
authors.initials: SP
authors.lastname: Haman
authors.forename: Richard A
authors.initials: RA
authors.lastname: Yeasting
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D001858:Bone Nails / Q000009:adverse effects*; D002102:Cadaver; D003399:Craniotomy; D016267:External Fixators / Q000009:adverse effects; D005260:Female; D006801:Humans; D008297:Male; D011183:Postoperative Complications / Q000517:prevention & control; D011475:Prosthesis Failure; D012886:Skull / Q000033:anatomy & histology / Q000000981:diagnostic imaging* / Q000601:surgery*; D014057:Tomography, X-Ray Computed; D019415:Torque