Screws : Wheeless' Textbook of Orthopaedics

- See: Lag Screws

- Discussion:
- screw most commonly used is the machine screw, threaded from the headto tip and with a blunt end;
- to insert these into bone, a preliminary drill hole must be made;
- following this, threads may be cut by a tap prior to the insertion of screw, or screw may be designed to cut its own
                   path w/ fluted tip;
- types of screws:
- 1.5 mm screws:
- 1.5 mm sized screw requires 1.1 mm drill bit
- used for phalangeal frx;
- 2.0 mm screws:
- 2.0 mm sized screw requires 1.5 mm drill bit
- used for phalangeal frx in larger patients;
- usef for metacarpal frx;
- 2.7 mm screws:
- used for metacarpal frx;
- 2.7 mm sized screw requires 2.0 mm drill bit
- AO 3.5 mm screw:
- core diameter of 2.4 mm and requires a 2.5 mm drill bit;
          - Cancellous Bone Screws: 4.0 mm
          - 4.5 mm cortical screw:
                - Bioabsorbable Screw Fixation in Coracoclavicular Ligament Reconstruction
          - Cancellous Bone Screws: 4.5 and 6.5 mm, malleolar screw made of stainless steel all have same core diameter of 3.0 mm;
- 3.2 mm drill bit is used to predrill thread holes for each of these screws;
- screw pitch:
- pitch of a screw is the distance between the threads, and the lead is distance thru which a screw advances with one turn;
- if screw has only one thread, the pitch and the lead are identical
- if screw has more than one thread, the lead of the screw is increased proportionally to the number of threads;
- double threaded screw has a lead double the pitch, and this allows the screw to be tightened more rapidly;
- tensile strength: (resistance to breaking)
- depends on root diameter (diameter of the screw between the threads)
- pull out strength:
- depends on the outside diameter of the threads;
- does not depend on the number of threads per inch has no effect on pull out strength of screw, provided 5 or 6 threads
                   are in cortex;
- shear strength:
- is proportional to the cube of the root diameter, and tensile strength is proportional to the square;
- tapping vs. non tapping screws:
- tap is designed in such a way that it is not only much sharper than thread of the screw, but also has a more efficient mechanism
                   of clearing bone debris, which therefore does not accumulate and clog its threads;
- recent investigative work has shown that in extremely thin layers of cortical bone, such as facial bones, self tapping screws
                   appear to have better holding power than the non self tapping screws of corresponding size;
- non self tapping screw is generally superior, except in extremely thin cortical bone, cancellous bone, and in flat bones such as
                   those of face, the skull, and the pelvis;
- it used to be thought that self tapping screws had weaker hold in bone;
- self tapping screw can be removed & reinserted w/o weakening its hold in bone provided it is carefully inserted;
- however, if inadvertently angled it will cut a new path and destroy already cut thread, which is a disadvantage;
- self tapping screws should therefore not be used as lag screws;