The presented method provides a means to produce paradigmatic incomplete burst fracture patterns in calf and human 4-FSU samples, which simulate injuries in human patients seen regularly in clinical practice. To our knowledge, this is the first technique capable of reproducibly induce incomplete burst fractures.
The presented protocol tries to simulate the described mechanism of Type A injuries of the thoracolumbar junction [7
]. According to Magerl's work, injuries are caused by axial compression of the spine with or without flexion with an almost exclusive effect on the vertebral body.
The presented procedure has taken advantage of the published classical approaches to study burst fractures, which utilized spine fragments, mounted onto a fracture apparatus. The apparatus induces fracturing by dropping a mass element on the spine specimen or high speed vertical compression by a hydraulic material testing apparatus [12
In addition, a distance-controlled mode of compression, which defines the impaction depth and velocity, was developed. Utilising this model, the used force is adapted to each specimen's resistance.
The presented protocol is limited by the required structural damage (temporary plate/screw fixation) to produce the desired fracture type in calf and human specimen. No relevant additional damage caused by the inserted screws after compression load was observed. However, this might be a limitation for further investigations.
Calf spine samples
Calf spines are commonly used specimens for biomechanical spine testing. Based on their biomechanical properties including motion range, calf spines are considered to be suitable specimens for implant systems and surgical procedures [14
]. Other studies have used calf vertebrae for investigating implant characteristics [15
]. However, important differences compared to human spines have been reported [14
]. Thus, several distinct features of calf spines should be taken into account.
In spite of anatomical similarities, Cotteril et al. [17
] found a greater length of the bovine spinous processes at distinct thoracic levels, and a greater length of transverse lumbar processes at L3 compared to human spines. These features might influence motion properties of calf spines. In addition, ligaments and muscle forces may play an important role [17
]. Especially in multi-segmental testing, Riley et al. reported significant differences in axial rotation and lateral bending [19
In addition, the metabolic parameters of calf vs. human spines warrant a critical view.
Swartz et al. found calf spines to be a suitable model for testing surgical implants and demonstrated that the values of equivalent mineral density (EMD) of 6- to 8-week old calf vertebrae match the density reported for young adult human vertebrae [18
]. However, no comparative data on endplate strength or cortical bone property of calf vs. human spine specimens is available. In our study, we used spines of 3- to 6-month old calves and no EMD or BMD values of the used calf spine samples have been recorded.
The age at which the calf spine appears to match the situation in adult human spine best is described as 6 to 8 weeks [14
]. A limitation in this study is the use of 3- to 6-month old calves due to restrictions on the availability. Further, the plating used for fracture production might weaken the adjacent vertebral bodies. This needs to be considered by using distinct implants in the future.
The presence of the physis and anatomical differences in immature bovine samples compared to human spines may also have influenced fracture induction. However, as long as the availability of human spine specimens is a limiting factor in conducting similar experiments, the bovine model is a helpful tool in spite of the discussed limitations and considerations.
Thus, being aware of these considerations, our protocol provides the possibility for interesting future work using calf spine samples in this incomplete burst fracture model.
Human postmortem samples
The objective of this study was not the evaluation of the required force to break a human or an immature bovine vertebra but the development of a reproducible method for further investigations. Thus, performing osteotomy-like lesions and distance-controlled compression were combined to modify classical protocols.
Kifune et al. [6
] revealed that up to 4.8 kN (57 Nm) is needed to break the human endplate. The recorded average failure load of 3.6 ± 1.3 kN in this study may be due to the utilised osteotomy-like endplate weakening or to possible differences in bone quality.
In contrast to most published protocols, five-segmental specimens have been used in this study. This may also have influenced the force required to induce the fracture.
Some authors have used a repeating dropping mass technique, a method that requires repeating the mass impact [5
]. Using our modified method, only a single compression event is necessary to generate the fracture.
Shono et al. used a high-speed vertical compression to inflict L1 burst fractures in multi-segmental specimens. Therefore, the L1 vertebra and adjacent discs have been isolated by upper and lower box-shaped fixtures. Compression was performed under displacement control in a compressive direction until the distance between the upper and lower fixture was reduced to 10% of the original height in 0.5 seconds [12
The axial compression of 20% of the original height of the target vertebra necessary in our protocol may have been due to a possible difference of rigidity of the used temporary fixation of the adjacent vertebrae.
However, the presented data imply that the use of a distance-controlled compression protocol provides excellent control in producing different fracture morphologies.
The same impact depth will be performed automatically even in spine specimens with more or less resistance so that the impact is automatically adapted to the used specimen. As indicated, ideal samples to study incomplete burst fractures would have been young human tissue. However, the presented technique resulted in incomplete burst fractures in osteoporotic human and young calf spine samples. This suggests that the presented technique might work on everything in-between and differences in bone quality may less influence the induction of similar injuries for biomechanical testing.
In our human samples, only minor differences in fracture morphology could be observed in specimens with different bone quality; thereby all fractures were rated as Magerl A3.1 fractures and a load sharing classification rating from 4 to 7 by an independent consultant radiologist and a senior spine surgeon.