TY - JOUR
T1 - Biomechanical testing of the LCP : how can stability in locked internal fixators be controlled?
AU - Stoffel, Karl
AU - Dieter, U.
AU - Stachowiak, Gwidon
AU - Gachter, A.
AU - Kuster, Markus
PY - 2003
Y1 - 2003
N2 - New plating techniques, such as non-contact plates,have been introduced in acknowledgement of theimportance of biological factors in internal fixation.Knowledge of the fixation stability provided by thesenew plates is very limited and clarification is still necessaryto determine how the mechanical stability, e.g.fracture motion, and the risk of implant failure can bestbe controlled. The results of a study based on in vitroexperiments with composite bone cylinders and finiteelement analysis using the Locking Compression Plate(LCP) for diaphyseal fractures are presented and recommendationsfor clinical practice are given.Several factors were shown to influence stability bothin compression and torsion. Axial stiffness and torsionalrigidity was mainly influenced by the working length,e.g. the distance of the first screw to the fracture site. Byomitting one screw hole on either side of the fracture,the construct became almost twice as flexible in bothcompression and torsion. The number of screws also significantlyaffected the stability, however, more thanthree screws per fragment did little to increase axial stiffness;nor did four screws increase torsional rigidity. Theposition of the third screw in the fragment significantlyaffected axial stiffness, but not torsional rigidity. Thecloser an additional screw is positioned towards thefracture gap, the stiffer the construct becomes undercompression. The rigidity under torsional load wasdetermined by the number of screws only.Another factor affecting construct stability was thedistance of the plate to the bone. Increasing this distanceresulted in decreased construct stability. Finally, ashorter plate with an equal number of screws caused areduction in axial stiffness but not in torsional rigidity.Static compression tests showed that increasing theworking length, e.g. omitting the screws immediatelyadjacent to the fracture on both sides, significantlydiminished the load causing plastic deformation of theplate. If bone contact was not present at the fracture sitedue to comminution, a greater working length also ledto earlier failure in dynamic loading tests. For simplefractures with a small fracture gap and bone contactunder dynamic load, the number of cycles until failurewas greater than one million for all tested constructs.Plate failures invariably occurred through the DCP holewhere the highest von Mises stresses were found in thefinite element analysis (FEA). This stress was reducedin constructions with bone contact by increasing thebridging length. On the other hand, additional screwsincreased the implant stress since higher loads wereneeded to achieve bone contact.Based on the present results, the following clinical recommendationscan be made for the locked internal fixatorin bridging technique as part of a minimally invasivepercutaneous osteosynthesis (MIPO): for fracturesof the lower extremity, two or three screws on either sideof the fracture should be sufficient. For fractures of thehumerus and forearm, three to four screws on either sideshould be used as rotational forces predominate in thesebones. In simple fractures with a small interfragmentarygap, one or two holes should be omitted on each side of the fracture to initiate spontaneous fracture healing,including the generation of callus formations. Infractures with a large fracture gap such as comminutedfractures, we advise placement of the innermost screwsas close as practicable to the fracture. Furthermore, thedistance between the plate and the bone ought to be keptsmall and long plates should be used to provide sufficientaxial stiffness.
AB - New plating techniques, such as non-contact plates,have been introduced in acknowledgement of theimportance of biological factors in internal fixation.Knowledge of the fixation stability provided by thesenew plates is very limited and clarification is still necessaryto determine how the mechanical stability, e.g.fracture motion, and the risk of implant failure can bestbe controlled. The results of a study based on in vitroexperiments with composite bone cylinders and finiteelement analysis using the Locking Compression Plate(LCP) for diaphyseal fractures are presented and recommendationsfor clinical practice are given.Several factors were shown to influence stability bothin compression and torsion. Axial stiffness and torsionalrigidity was mainly influenced by the working length,e.g. the distance of the first screw to the fracture site. Byomitting one screw hole on either side of the fracture,the construct became almost twice as flexible in bothcompression and torsion. The number of screws also significantlyaffected the stability, however, more thanthree screws per fragment did little to increase axial stiffness;nor did four screws increase torsional rigidity. Theposition of the third screw in the fragment significantlyaffected axial stiffness, but not torsional rigidity. Thecloser an additional screw is positioned towards thefracture gap, the stiffer the construct becomes undercompression. The rigidity under torsional load wasdetermined by the number of screws only.Another factor affecting construct stability was thedistance of the plate to the bone. Increasing this distanceresulted in decreased construct stability. Finally, ashorter plate with an equal number of screws caused areduction in axial stiffness but not in torsional rigidity.Static compression tests showed that increasing theworking length, e.g. omitting the screws immediatelyadjacent to the fracture on both sides, significantlydiminished the load causing plastic deformation of theplate. If bone contact was not present at the fracture sitedue to comminution, a greater working length also ledto earlier failure in dynamic loading tests. For simplefractures with a small fracture gap and bone contactunder dynamic load, the number of cycles until failurewas greater than one million for all tested constructs.Plate failures invariably occurred through the DCP holewhere the highest von Mises stresses were found in thefinite element analysis (FEA). This stress was reducedin constructions with bone contact by increasing thebridging length. On the other hand, additional screwsincreased the implant stress since higher loads wereneeded to achieve bone contact.Based on the present results, the following clinical recommendationscan be made for the locked internal fixatorin bridging technique as part of a minimally invasivepercutaneous osteosynthesis (MIPO): for fracturesof the lower extremity, two or three screws on either sideof the fracture should be sufficient. For fractures of thehumerus and forearm, three to four screws on either sideshould be used as rotational forces predominate in thesebones. In simple fractures with a small interfragmentarygap, one or two holes should be omitted on each side of the fracture to initiate spontaneous fracture healing,including the generation of callus formations. Infractures with a large fracture gap such as comminutedfractures, we advise placement of the innermost screwsas close as practicable to the fracture. Furthermore, thedistance between the plate and the bone ought to be keptsmall and long plates should be used to provide sufficientaxial stiffness.
U2 - 10.1016/j.injury.2003.09.021
DO - 10.1016/j.injury.2003.09.021
M3 - Article
SN - 0020-1383
VL - 34
SP - 11
EP - 19
JO - Injury: International Journal of the Care of the Injured
JF - Injury: International Journal of the Care of the Injured
IS - 2
ER -