Cerclage cable augmentation does not increase stability of the fixation of intertrochanteric fractures. A biomechanical study.

BACKGROUND: Intertrochanteric fractures with a posteromedial intermediate fragment are unstable because of the loss of medial support. Additional fixation with a cerclage is used in subtrochanteric fractures, but not in intertrochanteric fractures. The aim of this biomechanical study is to evaluate whether cerclage fixation improves stability of intertrochanteric fractures. HYPOTHESIS: Our hypothesis is that the cerclage fixation of the intermediate fragment increases fixation stability of intertrochanteric fractures. MATERIALS AND METHODS: Synthetic femora with intertrochanteric fractures (AO 31.A1.3) with a posteromedial fragment were fixed with a long gamma nail. The intermediate fragment was fixed with a cerclage cable. Four groups were compared: 1: no cable fixation; 2: anatomic reduction and cable fixation; 3: anatomic reduction and fixation of a fragment where its proximal part was removed simulating comminution; 4: non-anatomic reduction and cable fixation. The specimens were loaded axially in a testing machine. The preload was 100N, followed by ten conditioning cycles from 100N to 500N. The test phase consisted of the cyclic loading between 100N and the maximum force that increased at a rate of 50N at each cycle until failure. The stiffness was calculated from the load/displacement curve of the last three conditioning cycles. RESULTS: There were no statistically significant differences between force to failure (group 1: 681N; group 2: 846N; group 3: 699N; group 4: 806N; ANOVA p=0.23) and stiffness (group 1: 769N/mm; group 2: 819N/mm; group 3: 815N/mm; group 4: 810N/mm; ANOVA p=0.84) between groups. There were significant differences in the widening of the lag screw canal (group 1: 2.16mm; group 2: 4.5mm; group 3: 3mm; group 4: 2.5mm; ANOVA p=0.017). In individual comparison, the differences were significant only between the anatomical reduction group and the non-anatomical reduction (p=0.04) and the no cable group (p=0.02). DISCUSSION: There is a controversy in clinical literature whether cable fixation improves treatment outcome of proximal femoral fractures. This study suggests that medial wall reconstruction with a cerclage cable does not improve axial stability of the fixation. LEVEL OF EVIDENCE: Not applicable; a biomechanical study.

The cement-bone bond is weaker than cement-cement bond in cement-in-cement revision arthroplasty. A comparative biomechanical study.

This study compares the strength of the native bone-cement bond and the old-new cement bond under cyclic loading, using third generation cementing technique, rasping and contamination of the surface of the old cement with biological tissue. The possible advantages of additional drilling of the cement surface is also taken into account. Femoral heads from 21 patients who underwent a total hip arthroplasty performed for hip arthritis were used to prepare bone-cement samples. The following groups of samples were prepared. A bone-cement sample and a composite sample of a 6 weeks old cement part attached to new cement were tested 24 hours after preparation to avoid bone decay. Additionally, a uniform cement sample was prepared as control (6 weeks polymerization time) and 2 groups of cement-cement samples with and without anchoring drill hole on its surface, where the old cement polymerized for 6 weeks before preparing composite samples and then another 6 weeks after preparation. The uniaxial cyclic tension-compression tests were carried out using the Zwick-Roell Z020 testing machine. The uniform cement sample had the highest ultimate force of all specimens (n = 15; Rm = 3149 N). The composite cement sample (n = 15; Rm = 902 N) had higher ultimate force as the bone-cement sample (n = 31; Rm = 284 N; p <0.001). There were no significant differences between composite samples with 24 hours (n = 15; Rm = 902 N) and 6 weeks polymerization periods (n = 22; Rm = 890 N; p = 0.93). The composite cement samples with drill hole (n = 16; Rm = 607 N) were weaker than those without it (n = 22; Rm = 890 N; p < 0.001). This study shows that the bond between the old and new cement was stronger than the bond between cement and bone. This suggests that it is better to leave the cement that is not loosened from the bone and perform cement in cement revision, than compromising bone stock by removal of the old cement with the resulting weaker cement-bone interface. The results support performing cement-in-cement revision arthroplasty The drill holes in the old cement mantle decrease cement binding strength and are not recommended in this type of surgery.

The early failure of the gamma nail and the dynamic hip screw in femurs with a wide medullary canal. A biomechanical study of intertrochanteric fractures.

BACKGROUND: Intertrochanteric fractures may occur in a bone with a wide medullary canal that may lead to significant mobility of a intramedullary nail, contrary to an extramedullary device. This study evaluates the Dynamic Hip Screw and the gamma nail in AO 31.A2.1 fractures in these circumstances. METHODS: Synthetic femora with canals drilled to 18 mm were used. Five fixation types were examined: a 2 - hole and a 4 - hole Dynamic Hip Screw with a 2 - hole plate, a standard gamma nail with dynamic and static distal locking and a long gamma nail. The specimens were tested with cyclic axial loading, from 500 N increasing of 50 N increments in each cycle. Force at failure, overall stiffness, stiffness at the fracture site, location and mode of failure were recorded. FINDINGS: The short gamma nails dislocated into varus under preload because the nail migrated laterally. The Dynamic Hip Screw was initially stable, but some specimens rotated around the lag screw. The gamma nail was rotationally stable. Both implants failed through femur fracture. The long gamma nailed failed by screw cut - out at forces lower than the ultimate force of the short gamma nail. INTERPRETATION: This study shows that the gamma nail is unstable in a large medullary canal but offers better rotational stability of the proximal fragment. A modification of the nail design or the operative technique may be considered.