Intertrochanteric Fractures Fixed With Three Different Length Of PFNA：a Biomechanical Study

Objective: Following the population aging in social, the incidence ofintertrochanteric fracture is increasing gradually. Successful treatment ofintertrochanteric fractures is not only beneficial to patients’ health, but alsohelps to reduce the economic burden of social and personal. In recent years,more and more clinicians tend to choose intra-medullary implants for thetreatment of intertrochanteric fractures. Although intra-medullary implantshave more advantages in theory than extra-medullary implants, but they havemade an increased of femoral shaft fractures after their insertion. Manyresearchers believe that an intra-medullary implant with a longer stem maymore effectively redistribute loads to the distal end of intra-medullary implant,which can reduce the incidence of femoral shaft fractures. So we through theexperiment to observe three different length of PFNA fixed intertrochantericfractures, with the increase of fracture instability, the changes of the femoralstress distribution, and compare which the length of distal PFNA stressdistribution in the most reasonable.Methods: Twelve intact adult male femur of approximately equal lengthwere chosen to randomly divide into three groups. They were respectivelyfixed by PFNA which have three kinds of length, and the lengths were200mm,240mm and280mm. A total of eight strain gauges were placed in thedirectional of principal femoral strains on the medial and lateral surfaces ofeach femur. Each femur was held in a steel vice at15of adduction in thecoronal plane. The femurs were then subjected to successively increasingvertically applied compressive loads from0N to1200N using an electronicuniversal testing machine. Strain values were recorded by the digital straingauge. Each femur was tested under five types: Ⅰ.intact femur after insertionof PFNA, Ⅱ. with anexperimentally created two–part fracture, Ⅲ. with an three–part fracture with lesser trochanter fragment removed, Ⅳ.with an three–part fracture with lesser trochanter and a few medial cortex fragmentremoved, Ⅴ. with an three–part fracture with lesser trochanter and a mostmedial cortex fragment removed.Results: Strain values were recorded at gauge1-gauge4，which site inthe proximal femur of three groups of samples，decreased with decreasingfracture stability. Strain values recorded at gauge1which sites in medially justbelow the level of the lesser trochanter had the greatest reduction. when femurwith an three–part fracture with lesser trochanter and a most medial cortexfragment removed，strain values recorded at gauge1was reduced to almostzero.Strain values recorded at gauge6which distal to the level of the lockingscrew on the later sides was tension strain in the beginning in the short nailgroup and the medium-length nail group. When femur with an two–partfracture， tension strain were transformed into compression strain, andincreased with decreasing fracture stability. There had a significant differencebetweenⅠ-Ⅴ type（P=0.000），while no statistical significance betweenⅠ-Ⅳ type. Compression strain had significantly increase when femur with athree–part fracture with lesser trochanter and a most medial cortex fragmentremoved. Strain values recorded at gauge6was compression strain in thebeginning in the long nail group. There had no statistical significancebetweenⅠ-Ⅳ type in the long nail group（p=0.539）.Strain values recorded at gauge8which distal to the level of the distalend of the nail on the later sides was tension compression in the beginning inall groups，and increased with decreasing fracture stability. There had asignificant difference betweenⅠ-Ⅴ type（P=0.000）in the short nail group，the same to the medium-length nail group（P=0.002）. Compression strain hadno significantly increase between Ⅰ-Ⅲ type in the short nail group and themedium-length nail group. Compression strain had significantly increase whenfemur with an three–part fracture with lesser trochanter and a few medialcortex fragment removed in the short nail group，while compression strain had significantly increase when femur with an three–part fracture with lessertrochanter and a most medial cortex fragment removed in the medium-lengthnail group. There had no statistical significance betweenⅠ-Ⅳtype in thelong nail group（p=0.163）in the long nail group. Compression strain had notsignificantly increase from beginning to end in the long nail group.Conclusion: This study has shown that three length of PFNA has similarbiomechanical properties in the proximal femur， and they significantlydecreased the strain with decreasing fracture stability in the immediatesubtrochanteric region of the femur. In the same time，three length of PFNAall don’t significantly increase the strain in the region of the lock screw and thedistal end of the nail，when femur with an two–part fracture and with an three–part fracture with lesser trochanter fragment removed. Compression strainsignificantly increases at the lateral cortex of the locking screw and the distalend of the nail in the medium-length nail group，when femur with an three–part fracture with lesser trochanter and a most medial cortex fragmentremoved. Compression strain significantly increases at the lateral cortex of thelocking screw in the short group，when femur with an three–part fracture withlesser trochanter and a most medial cortex fragment removed. When femurwith an three–part fracture with lesser trochanter and a few medial cortexfragment removed，compression strain significantly increases at the lateralcortex of the distal end of the nail in the short group. Compression strain hasnot significantiy increase from beginning to end in the long nail group.Therefore Long PFNA should be choosed in order to decrease strain values inthe region of the lock screw and the distal end of the nail, to treat unstableintertrochanteric fractures with lesser trochanter and medial cortex fragmentremoved. In the treatment of stable intertrochanteric fractures, strain values donot significantly increase even if choosing common length of PFNA.