Biomedical Engineering Reference
In-Depth Information
vicinity of a potentially highly contaminated site. Masticatory stresses in human and animals
are nevertheless different and vary from one species to another according to their nutritional
habits. Models have been described in the rat, rabbit, dog, sheep and NHP.
3,27
As previously
stated by Hollinger and Kleinschmidt,
3
mandibular bone is very thin in rodents and rabbits
and retention of implants is challenging. We do not recommend mandibular resections in
these species. Mandibular discontinuity defects are preferably performed in dogs, sheep or
NHP in which they are easier to perform. Although objective information is lacking on the
subject, mandibular CSD is, according to Schmitz and Hollinger
2
probably as great as 40 mm
long in adult foxhounds and up to 25% the length of the mandible in NHP. Mandibular
resections are prone to postoperative infections and, when extensive, are challenging to stabilise.
Complications are minimised by (i) preventive extraction of teeth bordering the defect (maxil-
lary teeth included) 2 weeks before bone resection,
3
(ii) submandibular approaches to avoid
intraoral perforations, (iii) mandibular bone fixation with bone plates.
Orbital wall defects have been performed in sheep.
28
Model reproducibility may be supe-
rior to mandibular defects and morbidity lower but as they are located in nonloading bones
this model should only be used for material to be used in unloaded bones (Table 2).
Orthopaedic Surgery
Filling Defects
Many BRM are used as “bone fillers” in defects resulting either from bone sampling for
grafting procedures, from metaphyseal traumas, from surgical removal of benign or malignant
tumours. Bone fillers are engineered as paste-like, granular or massive (preset blocks) materials
and experimental defects must be compatible with these specific presentations.
Standardised metaphyseal defects have been described in rabbit
34
(Fig. 1) and sheep
35
distal
femoral condyles and in the dog proximal tibia.
36
They do not require additional specific
immobilisation and are highly reproducible thus allowing quantitative statistical analysis of
histological data. Defect lumens are sealed either with a bone flap or with methylmetacrylate
cement to prevent material leakage. As they are located in loaded bones, they are more interest-
ing than the CS Ilial defect recently described in the goat.
37
Segmental Bone Defects
Keys has shown that under experimental conditions, a segmental long bone defect 1.5 times
the diaphyseal diameter, exceeds the regenerative capacities of bone in skeletally mature dogs
and results in nonunion when bone loss is not replaced.
9
The length of bone to be resected to
obtain nonunion is species and bone-dependent and must therefore be established each time a
new model is designed (negative controls). Key's observations nevertheless roughly apply to
many species (cats, pigs, sheep, NHP) and can serve as guidelines for new designs (Table 3).
Rodents and rabbits remain exceptions to that rule as comparatively longer resections are needed
to generate nonunion in these species.
As stated previously, segmental defects must remain in a perfectly stable biomechanical
environment while healing occurs. Unlike in humans, weight bearing is difficult to prevent in
animals. Achille's tenotomy is not a reliable adjunctive technique as it only prevents
weight-bearing for a short time.
38
Models in which the bone fixation technique correctly
neutralises forces (locked intramedullary nailing, bone plating) are therefore preferred over
unlocked intramedullary nailing or external coaptation by plaster or casts which do not neutralise
compression, traction and rotation forces. Although satisfactory from a biomechanical point of
view, external fixation has several limitations: (i) weight bearing is inconsistent, (ii) bone lysis
around the pins occurs over time thus compromising stability, reducing model reproducibility
and relevance.
39,40
Femoral resections have been performed in rats by many authors: 5 to 8 mm long bone
resections (20 to 25% of femoral bone length) stabilised by custom made bone plates
4
or
external fixators
5
generate nonunion in approximately 12 weeks.
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