Vet Comp Orthop Traumatol 2020; 33(01): v
DOI: 10.1055/s-0039-1700982
Georg Thieme Verlag KG Stuttgart · New York

Drill, Measure, Tap and Screw: That's What AO Surgeons Know to Do

Kenneth A. Johnson
1  Department of Orthopaedics, Sydney School of Veterinary Science, University of Sydney, Sydney, New South Wales, Australia
› Author Affiliations
Further Information

Publication History

Publication Date:
21 January 2020 (online)

Zoom Image
Kenneth A. Johnson

Fixation of bone with screws or pins is the topic of several studies in this issue of the journal. Poor technique, amongst numerous other factors, can affect the accuracy and performance of screws and pins used in orthopaedic procedures. Surgeon-training at AO courses places a heavy emphasis on correct technique and correct use of the right instruments for screw placement, either as lag screws or plate fixation screws.

For the novice surgeon, the first step is to know the correct sequence to follow: drill, measure, tap and screw. However, even in the hands of experienced surgeons, complications can ensue during surgery or in the postoperative period due to screw failure, caused by technical mistakes or other undetermined reasons. From the outset, many factors may affect bone drilling for screw fixation, such as drill speed, wobble, blunt drill bit and bone density. But mechanical factors are not the only important ones, because there is also a biological cost of screw insertion, resulting in local regions of vascular disruption and bone necrosis.

Screw loosening or breakage is an oft-reported minor or major complication in clinical communications regarding internal fixation, including (for example) two of the reports in the current issue of the journal.

Behaviour of the surgeon performing the fixation procedure is a neglected consideration. For instance, there has been scant objective assessment of factors such as ergonomics of body position, instrument grip or alignment of the forearm during bone drilling. Experienced surgeons rely on auditory input and tactile feedback, during each step of the procedure of screw insertion. While drilling through bicortical bone, for example, the mid-diaphysis of the femur, experienced surgeons listen to the pitch of the advancing drill to judge when the full thickness of the trans-cortex has been traversed, thus minimizing the potential for serious soft tissue injury on the deep side of the bone by ‘plunging’ of the drill bit.[1] However, the performance of experienced orthopaedic surgeons in judging optimal insertion torque of 4.0 mm cancellous screws in a study using synthetic cancellous bone models was poor; 45% of the screws were tightened beyond the stripping torque and in the majority of cases this outcome was not recognized by the surgeons.[2] Optimal screw torque is low and more difficult for surgeons to judge by proprioception in osteoporotic bone.[2] [3] Moreover, there are large variations between surgeons in their ability to achieve optimal screw tightness, without stripping.[2] Fortunately experienced surgeons appear to demonstrate greater proprioceptive aptitude for achieving optimal screw tightness in dense cortical bone in which applied torques are about eightfold greater than those required in cancellous bone; in a study of 4.5 mm cortex screw insertion into cadaveric human femora and tibias using an instrumented screw driver, surgeons achieved an average of 86% of maximal torque (stripping torque), and less than 10% of the 4.5 mm screw were stripped.[3] A much better result.

While such studies highlighting sub-optimal performance of surgeons are humbling, it does also underscore the need for more research into the ‘human’ factors causing screw failure, and how the surgeon's sensory perception might be ‘sharpened’.