Horizontal oblique fractures of the medial malleolus occur through application of valgus or abduction force through the ankle that creates a tension failure through pull of the deltoid ligament. This occurs in injuries with abduction or valgus forces, such as supination-external rotation, pronation-external rotation, and pronation-abduction ankle fractures.1 Open reduction and internal fixation is indicated for unstable ankle fractures, having the primary goal of keeping the talus directly under the tibia, as any translation results in a significant alteration in joint biomechanics.2 Optimal fixation of these fractures must be sufficient to maintain stable anatomic reduction of the ankle joint's articular surface, allowing early range of motion, maintaining congruency of the ankle joint, and decreasing the risk of future posttraumatic arthritis to maximize functional outcome.3
A wide variety of techniques, including various configurations of cortical screws, cancellous screws, tension bands, and plates, are available for fixation of these fractures. Clinically, the fixation construct most commonly used for these fractures involves 2 parallel, 4.0-mm partially threaded unicortical cancellous screws placed perpendicular to the fracture line. This is only feasible when the medial malleolar fragment is large enough for 2 screws to be placed across it to achieve compression and rotational stability. Historically, a tension band construct was considered when the fracture was more complex or consisted of smaller fragments.4 Recently, a mini-fragment T-plate fixation was proposed, which was shown to have improved biomechanical properties compared with a unicortical cancellous screw or tension band construct and the ability to capture small fragments like a tension band, with the potential for decreased soft tissue complications seen with those constructs.5,6
Bicortical fixation of medial malleolar fractures with 3.5-mm cortical screws has also been proposed as an alternative to increase initial construct stiffness.7–9 This method significantly increases the strength of the fixation construct but still requires a fragment large enough for 2 large screws. Mini-fragment T-plate fixation with bicortical screws has the potential to be a superior fixation method, combining the advantages of the 2 techniques. The smaller 2.4-mm screws can be used in much smaller fragments, and making them bicortical may significantly increase the stiffness of the T-plate construct.
In this study, the authors evaluated the stiffness and force to 2-mm displacement of the joint surface of 4 different constructs in horizontal medial malleolar fractures in synthetic distal tibiae: a unicortical cancellous screw construct, a bicortical cortical screw construct, a mini-fragment T-plate construct with 2 proximal bicortical screws, and a mini-fragment T-plate construct with 2 distal bicortical screws. The authors hypothesized that the T-plate construct with distal bicortical fixation would withstand more load at 2-mm displacement than a T-plate with proximal bicortical screws, a bicortical cortical screw construct, or a unicortical parallel cancellous screw construct.
Materials and Methods
Identical horizontal osteotomies were made by a band saw in 40 left fourth-generation composite synthetic distal tibiae (Model No. 3401 Sawbones; Pacific Research Labs, Vashon, Washington) to simulate an OTA type 44-A2.3 fracture. Cuts were made with a custom jig set at a 15° angle from the tibial plafond, 10 mm from the distal tip of the medial malleolus, and directed toward the articular surface. After osteotomy, tibiae were randomized to 4 fixation groups, with 10 specimens in each group: (1) two parallel partially threaded unicortical 4.0-mm diameter 40-mm cancellous screws; (2) two parallel bicortical 3.5-mm diameter 50-mm cortical screws; (3) a contoured 2.4-mm mini-fragment T-plate with four 2.4-mm cortical locking screws (2 unicortical in the fragment and 2 bicortical in the shaft) (proximal bicortical); and (4) a contoured 2.4-mm mini-fragment T-plate with 2 bicortical locking screws in the fragment and 2 unicortical locking screws in the shaft (distal bicortical) (Figure 1). Custom poly(methyl methacrylate) jigs were used to reproducibly make identical holes. A 2.5-mm drill was used for the parallel unicortical and parallel bicortical screws constructs. A 1.8-mm drill was used for the T-plate constructs. For the T-plate constructs, a 2.4-mm plate with 7 holes in the shaft and 2 holes in the head with T-plates was precontoured so that the “T” portion of the plate cupped the medial malleolus and the linear portion of the plate fit intimately with the tibial shaft.
Photographs (A), anteroposterior radiographs (B), and axial radiographs (C) of the horizontal medial malleolus osteotomy in a synthetic distal tibia with each of the 4 fixation constructs (from left to right): parallel unicortical cancellous screw construct, bicortical screw construct, proximal T-plate construct, and distal bicortical T-plate construct. White lines indicate osteotomy cuts.
Specimens were fixed to the base of a servohydraulic testing machine (Model No. 809; MTS Systems Corporation, Eden Prairie, Minnesota) with an axial-torsional load transducer (Model No. 662.20-01; axial capacity of 250 kg and torsional capacity of 2.88 kg-m; MTS Systems Corporation). They were set in a vice with the distal tip of the medial malleolus offset 2 cm in the coronal plane from the axis of the testing machine's crosshead and at a vertical distance of 7 cm from the plafond. Two 1.5-mm solid steel wires were placed 8 mm from the tip and 10 mm apart in the distal medial malleolus fragment, placed over the crosshead of the testing machine, and held in place with U-bolt cable clamps to apply an offset axial tension load simulating the pull of the deltoid ligament in rotation ankle injuries that has been described previously (Figure 2).6 Tension was applied through the wires at 0.2 mm/s until failure of the construct, which was defined as 2 mm of articular displacement. Mode of failure was also noted. Load and axial displacement were measured at 60 Hz. Markers on the distal tibia and medial malleolus fracture fragment were tracked using high-resolution video (Fastcam PCI; Photron USA Inc, San Diego, California). Motion of the video markers was determined using digitization and motion analysis software (Motus 9; Vicon, Centennial, Colorado).
Offset axial servohydraulic tension of the medial malleolus simulating a pull of the deltoid ligament on the medial malleolus during rotational ankle injury. The medial malleolus was set 2 mm lateral to the axis of the testing machine's (Model No. 809; MTS Systems Corporation, Eden Prairie, Minnesota) crosshead and 7 cm away. Video motion tracking markers on the distal tibia and medial malleolar fragment are also visible.
Stiffness was calculated as the slope of the linear portion of the load-displacement curve over a range of 0.5 to 2.0 mm (Figure 3). Force at 2 mm of fragment displacement was defined as clinical failure.2,10 Both continuous variables were reported as mean (SD). A two-way analysis of variance followed by a Tukey–Kramer post hoc test was used to determine differences between construct stiffness and force for displacement at 2 mm. Statistical significance was defined as P<.05.
Overlaid representative plots of force vs displacement curves for the 4 fixation constructs: parallel unicortical cancellous screw construct (unicortical), bicortical cortical screw construct (bicortical), proximal bicortical T-plate construct (proximal T), and distal bicortical T-plate construct (distal T).
With offset axial tension loading to simulate pull of the deltoid ligament during a rotational ankle injury along with video motion analysis, the mean stiffness and force to 2 mm of articular displacement of both mini-fragment T-plate constructs and the bicortical screw construct were not statistically different (P>.05). The mean stiffness of the distal bicortical T-plate construct was 239±83 N/mm. The proximal bicortical T-plate construct had a mean stiffness of 190±37 N/mm. The bicortical screw construct had a mean stiffness of 240±17 N/mm (Figure 4A). These constructs were all significantly stiffer than the parallel unicortical screw construct (102±20 N/mm).
Bar graph showing stiffness (A) and load at 2 mm of displacement (B) with offset axial loading of each construct. Data are presented as mean, with error bars indicating SD. Statistical significance between indicated groups (*) is defined as P<.05.
A similar trend was seen with the force to clinical failure of these constructs. The mean force to 2 mm of articular displacement (SD) for the distal bicortical T-plate construct was 684±101 N. The proximal bicortical T-plate construct required 620±65 N. The bicortical screw construct required 672±57 N. The parallel unicortical screw construct required 392±34 N (Figure 4B). The proximal and distal bicortical T-plate constructs and the bicortical screw construct were not statistically different, but all withstood approximately 100% more force before clinical failure.
The failure modes of the constructs were also different. The parallel unicortical screw construct failed through screw pullout from the proximal segment. The bicortical screw construct failed through pullout of the distal medial malleolar fragment, while the screws stayed fixed in their distal aspect to the tibial shaft. The proximal T-plate construct failed with the proximal aspect of the medial malleolar fragment rotating medially along with screw pullout from the proximal fragment, while the distal T-plate construct failed through pullout of the distal medial malleolar fragment through the screws along with medial rotation. The T-plate did not break during failure of either construct.
The authors found that, when subjected to offset axial tension, a bicortical screw construct and 2 screw configurations of mini-fragment T-plate constructs exhibited significantly increased stiffness and load to 2 mm of displacement compared with a parallel unicortical screw construct. The increase in stiffness between unicortical and bicortical screw constructs has been reported previously.9 This was predicted by biomechanical studies of screws that showed increased pullout strength of bicortical screws11,12 and has been shown to be a clinically viable method for medial malleolar fixation with possibly improved outcomes.7,13 Amanatullah et al6 described an alternative method of medial malleolar fixation using a T-plate construct5 that was subsequently compared biomechanically with a unicortical screw construct, but all screws in the T-plate were unicortical. This is the first report comparing all methods and the first testing of a T-plate construct with multiple bicortical screw configurations. The authors' results indicate that the T-plate constructs provide initial fixation similar to that of a bicortical screw construct and significantly more initial stiffness. Therefore, the mini-fragment T-plate constructs provide a superior biomechanical construct, as well as a mechanism for capturing comminution that may not be captured by screws alone.
The authors made reproducible fractures and fixation in synthetic distal tibiae, which have less variability in size and quality than cadaveric bone. This allows reproducible measurements of construct stiffness and failure. The optimal stiffness of fixation to achieve union of medial malleolar fractures is unknown, so construct stiffness has been used as a surrogate marker for the quality of fixation methods.6,9,14
A limitation of this study included the use of synthetic rather than cadaveric bone. Fourth-generation sawbones have less geometric and structural variability than cadaver bone, allowing for more reproducible studies. They have biomechanical properties similar to those of real bone, although comparison studies have had variable results.15–17 Heiner15 estimated that fourth-generation tibia sawbones are approximately 15% to 28% less rigid than cadaver bone and have 30% less tensile strength. A review by Gardner et al17 comparing multiple studies showed that fourth-generation tibias may have the same to 16% increased rigidity compared with cadaver bone. Another limitation of this study is that it may not be applicable to osteoporotic bone, which would be significantly less dense than sawbones. This testing paradigm is also an artificial situation only designed to test construct stiffness and load to clinical failure in a single mode of stress, offset axial tension, with no attention paid to other possible modes of force. This testing setup does not take into account the structures surrounding the medial malleolus and tibia, such as the talus, fibula, or soft tissue attachments, such as the flexor retinaculum. These results are only relevant immediately after fixation, before bone healing occurs. The authors also tested load to clinical failure rather than cyclic loading. This testing more closely models a single traumatic force rather than the considerably smaller stresses that would be repeatedly exerted on the construct over several weeks after fixation in a clinical situation.
This was not a clinical outcomes study; rather, it suggests that mini-fragment T-plate constructs with bicortical screws are a viable and possibly superior method for the initial fixation of horizontal medial malleolar fractures. The effect of increased dissection required for application of a T-plate compared with unicortical screws in a clinical situation was not addressed. Also, although a T-plate construct has a lower profile than a traditional tension band, it may induce more soft tissue irritation than unicortical screws. Price is also a significant drawback to the T-plate construct. The list price for two 4.0-mm cancellous screws is approximately $70. The entire T-plate construct costs at least 10 times more. Although most implants are purchased at a discount from the list price, the cost of T-plate constructs may be prohibitive.
Two configurations of mini-fragment T-plate fixation of horizontal medial malleolar fractures were found to have stiffness and force to clinical failure equivalent to that of a bicortical screw construct, and all were found to be superior to a unicortical cancellous screw construct. Although the most commonly used construct for medial malleolar fixation continues to involve unicortical cancellous screws, this study adds to an increasing body of evidence that alternative fixation techniques provide advantages over that construct, including increased initial stiffness and the ability to capture comminuted fragments that would not be captured by screw constructs alone.
Contoured mini-fragment T-plate constructs with either proximal or distal bicortical screws provide stiffer initial fixation than a unicortical cancellous construct. The T-plate is biomechanically equivalent to a bicortical screw construct, but may be superior in capturing small fragments of bone.
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