Purpose: Locked plating has become a commonly used technique in complex fracture and nonunion work. The combination of locked and unlocked screws in the same construct has been referred to as “hybrid” fixation. Little work is available to direct the specifics of this fixation method. The purpose of this study was to determine the relative contribution of the number and location of locked screws on the properties of hybrid plate constructs in an osteoporotic bone model.

Method: A prefabricated osteoporotic model was used for reproducibility (composite cylinders 35 mm in diameter and consisting of a 2.5 mm fiberglass shell filled with 10 lb/ft3 polyurethane). A 5mm gap model was used, and fixed with a 12 hole plate. Six different constructs were tested including 2 unlocked and 4 hybrid configurations. All screws were bicortical and placed with 4Nm of torque. Baseline removal (loosening) torque was recorded for each screw for comparison with removal torque after cyclic loading. Testing was performed with ±8Nm of torsional load and run to 100,000 cycles. Stiffness of each construct was measured at 10,000 cycle increments and the removal torque of each screw was recorded at the conclusion of the 100,000 cycles.

Results: Stiffness of the constructs was most affected by the number of screws. No effect was seen with the replacement of one or two unlocked screws with locked screws on each side of the gap. Replacement of three unlocked screws with locked screws increased the stiffness of the construct (p< 0.001).

Conclusion: At least three bicortical locked screws on each side of a construct are needed to increase the stiffness and decrease the loss of stiffness over 100,000 cycles of torsional stress in an osteoporotic surrogate model. Locked screws placed between the fracture and unlocked screws protect the unlocked screws from loosening and may have some clinical utility in fatigue of the construct.

In recent years, plate osteosynthesis in metaphyseal and diaphyseal long bone fractures has been increasingly applied in a minimally invasive fashion. Several clinical studies describe a beneficial effect of the smaller additional soft tissue damage, resulting in satisfying fracture and soft tissue healing. However, is the surgical soft tissue damage really evidently smaller and the recovery faster?

A trauma model with severe, circumferential soft tissue damage to the distal right thigh and a distal multifragmentary (AO type C) femur fracture was carried out on 24 male sheep. After five days temporary external fixation, an internal fixator was placed either by a conventional open lateral approach or by minimally invasive technique. To assess the soft tissue damage and its recovery within the first 14 days, local compartment pressure monitoring as well as daily measurements of systemic markers (Creatin Kinase, CK and Lactate Dehydrogenase, LDH) in blood were performed. The local monitoring with a special probe (Neurovent PTO, Raumedic AG, Germany) within the quadriceps muscle allowed the measurement of compartment pressure (CP), as well as temperature.

The CK and LDH levels responded to the severe trauma with high peaks within the first 48 hours post trauma. After the internal fixator operations CK levels illustrate a significantly lower increase (p< 0.05) in the minimally invasive group compared to the open approach group in the first two days postoperatively. LDH levels show lower values for the minimally invasive group (p=0.06).

The values of CP present an initial increase after the trauma and then higher values (p=0.08) after the open plating operation. For the intracompartmental temperature no statistical differences were found, too (p=0.17).

These results, with reduced additional soft tissue damage and faster recovery in the minimally invasive approach group, reflect the clinical experience and expectations. However, while minimally invasive plate osteo-synthesis is certainly a desired option for fracture fixation, good surgical skills are required to insure that the reduced surgical trauma is in line with optimal fracture healing. The influence of the two different approaches on the bone healing per se, as well as the influence on soft tissue functionality, has yet to be demonstrated.