Background
Fractures of the femur at or just above the knee (condylar and supracondylar) comprise 4–7% of all femoral fractures and are considered to represent a challenge to treat
The Retrograde Biomet Matthews Nail
The Retrograde Biomet Matthews Nail.
Saggital view of the "cruciate" orientation of the distal locking screws in the femoral condyles
Saggital view of the "cruciate" orientation of the distal locking screws in the femoral condyles.
Axial view of the "cruciate" orientation of the distal locking screws in the femoral condyles
Axial view of the "cruciate" orientation of the distal locking screws in the femoral condyles.
A preliminary report by Henry et al
The objective of this study was to assess the comparative mechanical endurance performance of the new Biomet Matthews Nail and the Dynamic Condylar Screw, when dynamically tested in Y fractured femurs and transverse osteotomised composite femurs.
Methods
The intramedullary nails and dynamic compression screws were implanted into composite femurs which were subsequently cyclically loaded using a materials testing machine. Simulated fractures were applied to each femur prior to the application of the load. Either a Y type osteotomy or a transverse osteotomy was prepared on each composite femur using a jig to enable consistent positioning of cuts.
Design of the Biomet Matthews Nail
The intramedullary nail is a solid stainless steel rod available in two diameters; 12 and 14 mm and four different lengths; 200, 250, 300 and 350 mm. The nail has a single radius of curvature of 2290 mm. The geometry of the nail permits its use for both left and right femurs (Figure
The distal locking screws pass through the femoral condyles, posterior to the intramedullary line, in a unique "cruciate" orientation to achieve maximum bone purchase (Figure
Test specimens
• Eight Biomet Matthews Nails (12 and 14 mm diameters). The two sizes differed only in their diameter superior to the distal locking screw holes. Therefore nail performance, under test, in the distal screw region was considered to be equivalent for both sizes.
• Four 95 degree suprcondylar plates (4 off Supraloc CCS Plate)
• Four Lag screws (4 off Hiploc CHSS lag screw 12.5 Diameter)
• Biomet ST-PRO fixation screws (6 mm × 35 mm: 6 mm × 40 mm: 6 mm × 45 mm:)
• Biomet Cancellous bone screws (6.5 mm × 32 mm thread, length = 40 mm)
Implantation of nails and plates into composite femurs
The nails and dynamic compression screws were implanted into left composite femurs (Sawbones Europe AB, Malmö, Sweden) by MGM using the appropriate sets of instruments. Fixation screws were inserted through the nails, using the two most inferior cross-holes. Fixation screws were secured through each hole of the plates except for the hole immediately superior to the lag screw, which received a cancellous bone screw. Radiographs of the femur were taken following the insertion of each fixation device, to confirm their correct placement.
Cutting of Y fractures & transverse osteotomies into composite femurs
The composite femoral cuts were determined using the AO classification, one of the most commonly used classifications for distal femoral fractures
All nails, plates and screws were removed from individual femurs prior to any cuts being made. A femur would then be clamped within a custom made bone cutting jig and cuts made using either a Y fracture guide or an osteotomy guide. Figure
Photograph demonstrating the transverse osteotomy and Y fracture patterns
Photograph demonstrating the transverse osteotomy and Y fracture patterns.
The use of a Y fracture resulted in the condyles being secured only by the fixation device, which therefore experienced considerable loading. Similarly, an osteotomy was used (10 mm wide) to enable the entire load to be transmitted through the nail or the plate. Following cutting all implanted items were reinserted as performed previously.
Test procedure
Testing was carried out using a servo-hydraulic materials testing machine (Dartec HC 10, Dartec Ltd.) of 10 kN capacity (National Association Measuring Analysis Systems calibrated). This has a top cross-head mounted actuator, load cell and linear voltage displacement transducer. Femurs were positioned vertically within the test machine with the femoral head uppermost. The femoral condyles were placed on a Denham knee tibial component (Biomet Ltd.), supported in a cradle, which balanced on a knife edge (Figure
Photograph demonstrating the method of condyle support
Photograph demonstrating the method of condyle support.
The knife edge was positioned to allow the tibial component to be free to balance about an anterior-posterior axis, passing over the surface of the tibial bearing. An ultra-high molecular weight polyethylene cylinder, or loading socket, was positioned between the femoral head and the test machine actuator. The lower face of the cylinder was concave to produce a congruent fit with the femoral head. Thus the design of the tibial support ensured that load applied through the femoral head was transmitted equally through both femoral condyles (Figure
Photograph demonstrating the femur within the test machine
Photograph demonstrating the femur within the test machine.
Test conditions and data collection
Sinusoidal loading was applied to each specimen between specified maximum and minimum compressive loads at a frequency of 5 Hz, until 1 million cycles had been executed or specimen failure had occurred (whichever was sooner).
The four groups of test specimens consisted of:
1. BMN in femurs with Y osteotomies
2. BMN in femurs with transverse osteotomies
3. DCS on femurs with Y osteotomies
4. DCS on femurs with transverse osteotomies
Maximum and minimum vertical displacement and vertical displacement amplitude were generally logged twice daily over the course of the 56 hours, or less, as required for each test.
Results
Endurance Testing of the Biomet Matthews Nail and Dynamic Compression Screws tested with Y fractured composite femurs (Table
Endurance testing of the Biomet Matthews Nail and the Dynamic Compression Screw tested with Y fractured composite femurs
Biomet Matthews Nail with Y fractured femur
Dynamic Compression Screw with Y fractured femur
Max. & Min. of Load Cycle (kN)
Specimen Number
Cycles Completed
Specimen Number
Cycles Completed
2.0 & 0.2
1
1,000,000
1
1,000,000#
2.0 & 0.2
2
315,043
2
1,000,000#
3.0 & 0.3
3
22,356+
3.0 & 0.3
4
78,386
# Post test examination revealed a fracture through the second hole, superior to the lag screw
+Lateral condyle bone segment shifted laterally until purchase on the distal locking screw became sufficient to bend it
Biomet Matthews Nails
Of the two nail specimens tested at 2.0 kN (max. of cycle) specimen, 1 was a run out (end of test reached without failure), whilst specimen 2 failed after 315,043 cycles. The failure resulted from the lateral locking screw fracturing at the distal point of entry into the nail.
Two specimens were tested at 3.0 kN with both failing in different ways; the failure of one specimen at 22,356 cycles resulted from the lateral condyle bone segment shifting laterally until the purchase on the distal locking screw became sufficient to bend it (Figure
Photograph demonstrating a bent lateral locking screw with nail (Specimen 3)
Photograph demonstrating a bent lateral locking screw with nail (Specimen 3).
Photograph demonstrating a fractured distal locking screw with nail (Specimens 4)
Photograph demonstrating a fractured distal locking screw with nail (Specimens 4).
Dynamic Compression Screw
Two specimens were tested at 2.0 kN with failure occurring in both, through the 2nd plate hole (Figures
Photograph demonstrating a fracture through a screw hole with a screw in-situ
Photograph demonstrating a fracture through a screw hole with a screw in-situ.
Photograph demonstrating a fracture through a screw hole with the screw removed
Photograph demonstrating a fracture through a screw hole with the screw removed.
Endurance testing of the Biomet Matthews Nail and the Dynamic Compression Screw with transverse osteotomised composite femurs (Table
Endurance testing of the Biomet Matthews Nail and the Dynamic Compression Screw tested with transverse osteotomised composite femurs.
Biomet Matthews Nail with transverse osteotomised femur
Dynamic Compression Screw with transverse ostetotomised femur
Max. & Min. of Load Cycle (kN)
Specimen Number
Cycles Completed
Specimen Number
Cycles Completed
1.50 & 0.15
-
-
1
26,717
1.50 & 0.15
-
-
2
357,522
2.0 & 0.2
1
1,000,000
3.0 & 0.3
2
1,000,000
4.0 & 0.4
3
~14,500*
4.0 & 0.4
4
~21,000*
* Test aborted due to deterioration of femoral head in loading socket
Biomet Matthews Nail
Neither of the two specimens tested at 2.0 and 3.0 kN (max. of cycle) failed. This was also the case at 4.0 kN, but both tests at this load level had to be aborted at approximately 14,000 cycles and 21,000 cycles, due to the deterioration of the femoral head in the loading socket.
Dynamic Compression Screw
Two specimens were tested at 1.5 kN, with failure occurring in both at close to 30,000 cycles. This occured through the plate hole at the level of the osteotomy.
Discussion & Conclusion
The insertion of an intramedullary nail is seen as a less technically demanding procedure than a dynamic compression screw
In an early clinical study using supracondylar intramedullary nails in distal femoral fractures Innacone et al
A study using composite femurs by Firoozbakhsh et al demonstrated superior rigid stability with the DCS, yet the intramedullary nail performed well enough to be considered as a reasonable biomechanical alternative in clinical practice. Since then many new designs of retrograde intramedullary nails have been used in clinical practice and it is now a well recognised procedure to treat distal femoral fractures
A limitation of our study was the number of modes of loading used. In Firoozbakhsh's study the devices were tested in six different modes of loading including compression in varus and valgus, axial torsion, medial and lateral bending and bending in flexion. We also tested the devices using composite femurs but not on human cadaveric bone.
Zlowodzki et al
These previous studies looked at the biomechanical stability of the supracondylar (extra-articular) fractures. However the most challenging fracture for the surgeon to treat is the Y shaped bicondylar (intra-articular) fracture
Competing interests
MGM holds the patent for this device and is working in conjunction with Biomet in its production. PS works for Biomet Ltd UK as an engineer.
Authors' contributions
LOR, JD & BD reviewed the results, published literature and co-wrote article. MGM designed the Biomet Matthews Nail and implanted the nails and dynamic compression screws. PS performed the mechanical testing of the devices and co-wrote the article. All authors have read and approved the final manuscript.