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Biomechanical analysis of stability of joint depression calcaneal fractures: Fixation with locking compression plate alone compared with addition of supplemental oblique screw
Allegheny Health Network, Department of Orthopaedic Surgery, Pittsburgh, PA, 15212, USAUniversity of Pittsburgh, Department of Mechanical Engineering and Materials Science, Pittsburgh, PA, 15213, USA
Allegheny Health Network, Department of Orthopaedic Surgery, Pittsburgh, PA, 15212, USAUniversity of Pittsburgh, Department of Mechanical Engineering and Materials Science, Pittsburgh, PA, 15213, USA
The standard treatment of calcaneus fractures is a lateral plate and screw construct. Patients at our institution have been treated with a lateral locking plate combined with one retrograde screw inserted in the oblique plane to allow immediate weight bearing. The purpose of this study was to determine whether addition of a oblique screw to a lateral plate construct increases stability.
Methods
A Sanders 2B fracture (AO/OTA 83-C2) was created in 8 pairs (16 total specimens) of cadaveric feet. All were repaired using a lateral locking plate/screws construct. One specimen in each pair was chosen randomly to receive an additional oblique screw. The specimens were tested with cyclic load of up to 800 N. Movement at the fracture sites and subsidence of the talus were tracked with a three-dimensional video analysis system.
Results
Talar subsidence was not significantly affected by the presence of the additional oblique screw (p = 0.22). The sustentaculum fragment in the case of the screw repair moved 0.39 mm while the same fragment without the additional screw repair displaced 0.12 mm (p < 0.01). Two repairs with and one repair without the additional screw failed during longer-term cyclic loading.
Conclusion
The two repair types were not statistically different in regards to talar subsidence. While statistical significance resulted in the comparison of sustentaculum fragment movement, the amount of movement did not reach a level of clinical relevance. This study demonstrated immediate stability and durability of the additional screw construct with high volume weight bearing loads.
1. Introduction
The standard of care for surgical treatment of calcaneus fractures is plate osteosynthesis through a lateral approach. While successful, patients maintain non-weight bearing status for several weeks, leading to loss of bone mass and muscle atrophy, with studies showing a loss of mineral density and of bone at lower extremity fracture sites up to 52 weeks after injury.
Restricted weight bearing has also shown functional limitations secondary to altered gait mechanics and upwards of a four-fold increase in the energy expended for ambulation.
Lin et al. compared the stability of lateral plate constructs with screws out of the plate vs all screws within the plate to capture fracture fragments; they showed having all screws within the locked plate construct was superior.
However, all screws in this study were oriented from lateral to medial. Rausch et al. studied plate osteosynthesis with cement augmentation of screw fixation and showed that cement augmentation fixation provided increased stability.
A biomechanical comparison of fixed angle locking compression plate osteosynthesis and cement augmented screw osteosynthesis in the management of intra articular calcaneal fractures.
These augmented screws, however, were placed in the anterior-posterior or medial-lateral plane. In recent clinical scenarios, calcaneal fracture patients with joint-depression at our institution have been treated with lateral locking plate combined with one or two fully threaded screws inserted in the oblique plane through the plantar aspect of the calcaneus. All patients began weight bearing immediately post-op and went on to successful osteosynthesis [Fig. 1].
Fig. 1A: Lateral calcaneal plain radiograph of split depressed calcaneus fracture with lateral locking plate fixation. B) Lateral calcaneal plain radiograph of plate fixation with stand alone obliquely oriented screw from distal and posterior to anterior and proximal.
The purpose of this study is to investigate whether the addition of an oblique screw to a lateral plate will result in increased stability of calcaneus fractures, especially in the posterior facet, in comparison to conventional lateral plating alone, using cadaveric bones with a Sanders type 2B fracture. This study aims to determine if the novel method infers more stability and allows earlier weight-bearing.
2. Methods
The talus and calcaneus were removed from eight pairs of cadaveric feet (16 specimens) and a known fracture model [Lin et al. CORR 352. 1998.] (Fig. 2) was created in the calcaneus and repaired using a lateral locking plate and screws (Arthrex, Naples, FL). One specimen in each pair was randomly assigned to receive an additional screw in the inferior, posterior calcaneus traveling anterior-superior at approximately 45° from the horizontal. A custom box-like fixture was filled with polyester filler (3 M, St. Paul, MN) and the superior talus was molded into the free surface before the resin hardened. The talus and calcaneus were then mounted in a custom three-point bending fixture (Fig. 3) adapted to a tensile testing machine (MTS Bionix 858, Eden Prairie, MN). The three points of loading were the extreme anterior and posterior sections of the inferior calcaneus and the superior surface of the talus. Congruence of the subtalar joint and physiologic orientation of the talus and calcaneus were ensured through the placement of the talus and the configuration of the three-point bending fixture. Optical markers for camera tracking (Spicatek DMAS, Kihei, HI) were attached to the lateral sides of the talus, posterior calcaneus, and anterior calcaneus (Fig. 4). Talar subsidence was calculated as the movement of the talar markers towards the average of the posterior and anterior calcaneal markers. Opening of the fracture repair sites was similarly tracked by pairs of markers on either side of the repaired bone (Fig. 4). Opening of the fracture beneath the talus was measured at the end of each set of cyclic loading using precision calipers, recording the distance between small holes drilled in the bone's surface.
Fig. 2The calcaneus with a known fracture model [Lin et al. CORR 352. 1998.] repaired with a lateral plate and screw construct.
Fig. 4(a) Lateral and (b) superior views of the calcaneus fracture (red dotted lines) model. Left to right is posterior to anterior on both subfigures. The locations of the tracking markers (M1 etc.) are shown. M1 is located on the lateral talus. The measured displacements are (1) M2-M3; (2) M1-(M2+M3)/2; (3) M4-M5; (4) M6-M7; (5) M8-M9.
The specimens were cyclically loaded in eight sets of 100 cycles at 1 Hz with increasing loads. For all sets, the minimum load of a given cycle was 20 N. The peak load in each set increased by 100 N up to a maximum of 800 N. The maximum loading of 800 N was chosen to replicate loading during standing. After the eight 100 cycle sets, a further 3000 cycles at 800 N were applied to examine short-term slow ambulatory loading.
Five measurements (Fig. 4) were performed after each 100 cycles and after the 3000 cycles of loading to answer the following three questions: 1) Do the calcaneus fragments spread apart in the anterior-posterior direction as the talus subsides: Measurement 1, M2-M3; 2) Does the talus subside: Measurement 2, M1-(M2+M3)/2; and 3) Do the gaps at fracture sites increase beneath the talus: Measurement 3, M4- M5; Measurement 4, M6-M7; Measurement 5, M8-M9.
Five two-way repeated measures ANOVAs with factors of repair type (two of repair types: no additional screw, i.e. control, and additional screw) and load condition (three of load conditions: 100 cycles of 100 N, 100 cycles of 800 N, and 3000 cycles of 800 N) were used to compare the five measurements. Significance was set at p < 0.05. Any failure modalities were recorded.
3. Results
Similar talar subsidence occurred for both repair types (p = 0.22, Table 2), but load condition effects were significantly different (p < 0.01): loading of 800 N after both 100 cycles and 3000 cycles produced significantly more displacement than after 100 cycles of 100 N. Both repair type and load condition showed significance for sustentaculum fragment diastasis (p < 0.01 for both): the additional screw condition showed more displacement than the control condition and there was more displacement after 3000 cycles of 800 N than after 100 cycles of 100 N. Averaged across the load conditions, the sustentaculum fragment in the case of the screw repair type moved 0.39 mm while the same fragment in the control repair type displaced 0.12 mm (Fig. 5). No significant differences were found among the remaining displacements for repair type, load condition, or their interaction (Table 1). Among all measurements taken (Table 2), the largest displacement, 1.33 ± 0.92 mm, was of sustentaculum fragment displacement for the screw repair type after 3000 cycles of 800 N. Two repairs with the additional screw and one repair without the additional screw failed. All three failures occurred during the 3000 cycles of 800 N.
Fig. 5The displacement of the sustentaculum fragment, averaged across all load conditions. p = 0.22.
Table 1p-values for the five two-way repeated measures ANOVAs calculated on the five measurements (1) M2-M3; (2) M1-(M2+M3)/2; (3) M4-M5; (4) M6-M7; (5) M8-M9). Factors were repair type and load condition. Significance was set at p < 0.05. Significant findings were shaded in gray.
Table 1p-values for the five two-way repeated measures ANOVAs calculated on the five measurements (1) M2-M3; (2) M1-(M2+M3)/2; (3) M4-M5; (4) M6-M7; (5) M8-M9). Factors were repair type and load condition. Significance was set at p < 0.05. Significant findings were shaded in gray.
Table 2Displacements for all five measurements (1) M2-M3; (2) M1-(M2+M3)/2; (3) M4-M5; (4) M6-M7; (5) M8-M9) as a function of repair type and load condition (mm).
This study of a standard calcaneal fracture pattern compared two forms of fixation: one type with an additional oblique screw to the standard lateral plate's screws toward the subtalar joint and one with the usual lateral plate and screw hardware.
Biomechanical comparison of conventional and anatomical calcaneal plates for the treatment of intraarticular calcaneal fractures - a finite element study.
A biomechanical comparison of fixed angle locking compression plate osteosynthesis and cement augmented screw osteosynthesis in the management of intra articular calcaneal fractures.
and thus a locked plate construct was the baseline for this study. The comparison was performed analyzing talar subsidence as the primary outcome and calcaneal lengthening and fracture fragment diastasis as secondary outcomes. The two methods were not found to be statistically different in regards to talar subsidence, but a small displacement of the sustentaculum fragment was significantly greater for the additional screw repair. The displacements when comparing the two different loads were on the order of fractions of millimeters, and the clinical importance of these displacements would be small. With only three total construct failures, both fixation methods showed durability with larger numbers of weight bearing loads. Plantar subsidence, loosening of fixation and any fragment fractures were the primary focus of the current study.
They noted failures as any specimens with loss of fixation of the sustenaculum tali fragment due to screw penetration. This failing was not due to either the presence or absence of the additional screw and would be potentially important factor in all repairs. If the screws are radiographically seen to only minimally hold the fragment, weightbearing under any circumstances might cause lead to fragment migration and non-union.
This study had several limitations. The comparison only measured talar subsidence using tracking of movement on the lateral face of the calcaneus. Opening of the fracture site beneath the talus did quantify any splaying of the bones with talar subsidence, but did not directly quantify plantar movement of the bone. An additional limitation was the omission of any specific complete failure test. Failure was tracked with talar subsidence and increasingly larger loads could have flattened the calcaneus completely without any fractures of the fragments. Complete failure testing was intentionally omitted because no clear failure criterion was readily available. Continued increases in load after the 3000 loading cycles led to compression of the calcaneus with no identifiable failures of either the plates or bones.
In conclusion, a common calcaneal fracture with multiple fragments repaired with a plate and with screws parallel to the fracture planes can be supplemented with an oblique screw to the fracture planes to support the subtalar joint. This study did not find a statistically significant benefit to the additional screw in talar subsidence, but it did demonstrate prolonged durability of the construct throughout a high volume of weightbearing loads. Testing of more specimens including a frequency of failure analysis should be conducted to see if this additional screw fixation has a prolonged time to failure when compared to lateral plate and screw construct.
References
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Changes in bone mass and bone turnover following ankle fracture.
Osteoporos. Int. J. Establ. Result Coop. Eur. Found. Osteoporos. Natl. Osteoporos. Found. USA.1999; 10: 408-415
A biomechanical comparison of fixed angle locking compression plate osteosynthesis and cement augmented screw osteosynthesis in the management of intra articular calcaneal fractures.
Biomechanical comparison of conventional and anatomical calcaneal plates for the treatment of intraarticular calcaneal fractures - a finite element study.
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