- See:
-
Cortex Screw: 3.5 mm
-
Cancellous Bone Screws: 4.0 mm
-
Cancellous Bone Screws: 4.5 and 6.5 mm
-
4.5 Cortical AO Screw:
-
Lag Screws:
-
Malleolar 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;
-
4.5 mm cortex screw,
6.5 mm cancellous bone screw, &
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;
- experimental evidence has shown that 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;
- Cannulated Screws:
- as noted by Hearn et al, the extraction strength of cannulated screws is not significantly affected by the changes needed to accomodate cannulation;
- references:
The Holding Strength of Cannulated Screws Compared with Solid Core Screws in Cortical and Cancellous Bone;
R. Leggon, R.W. Lindsey, B.J. Doherty, J. Alexander, and P. Noble. J. Orthop Trauma. Vol 7, No. 5, p 450-457.
Extraction Strength of Cannulated Cancellous Bone Screws. T.C. Hearn, J. Schatzker, and N. Wolfson. J. Orthop Trauma 1993. Vol 7. No 2. p 138-141.
A study of some factors which effect the strength of screws and their insertion and holding power in bone. Ansell H, Scales JR. J Biomech 1968. 1:279-302.
The mechanical properties of surgical bone screws and some aspects of insertion practice. Hughes AN, Jordan A. Injury 1972. 4:25-38.
The holding power of orthopaedic screws in vivo. J. Schatzker, R. Sanderson, JP Murnaghan CORR 108: 115-126. 1975.
A comparison of fixation screws for the scaphoid during application of cyclic bending loads. EB Toby et al. JBJS. Vol 79-A No 8. Aug 1997. p 1190.
Optimizing bone screw pullout force. TA DeCoster et al. J. Orthop. Trauma. Vol 4. 1990. p 169-174.
Effect of screw torque level on cortical bone pullout strength.
Thermal osteonecrosis and bone drilling parameters revisited
The effect of pilot hole size on the insertion torque and pullout strength of self-tapping cortical bone screws in osteoporotic bone.
