The Utilization Of Trabecular Metal In Combination With Autgraft Bone In Acetabular Reconstruction: A Preliminary Sheep Model
A Nabavi, J Field
A Nabavi, J Field. The Utilization Of Trabecular Metal In Combination With Autgraft Bone In Acetabular Reconstruction: A Preliminary Sheep Model. The Internet Journal of Orthopedic Surgery. 2008 Volume 13 Number 2.
Primary or secondary acetabular bone loss, at the time of initial total hip replacement
(developmental dysplasia), or revision of actebalular components with cavitatory
defects in situ, continue to be problematic. Currently, there are three preferred
techniques used when dealing with Type III (1-4) acetabular defects; these include
over-reaming and the utilization of a larger cup (5,6), the use of a cage in combination
with a cemented cup (7,8) or impaction grafting techniques (9-15).
The use of large revision cups, although technically easier, has some major
disadvantages including a net loss of bone stock which is further complicated by
stress shielding exacerbated by the larger, stiffer cups used (5,6). The utilization of
retainment cages in combination with cement has been reported to have a 20% failure
rate at 5 years (16,17). Likewise, impaction grafting has problems including cup
‘spin-out’ because of a failure to integrate at the bone graft-cement interface (14,16).
Concurrent problems include inadequate initial stability and difficulty controlling
cement intrusion into the bone graft. However, the advantages include augmentation
of bone stock and a restoration of the centre of rotation of the hip (17-19).
Slooff et al (1984) first reported the successful utilization of impaction grafted bone in
acetabular reconstruction; a 90% survival was achieved at 10-15 years (10). The
Exeter group (20) have been unable to match these results in a study following 111
acetabular reconstructions in which grafting was utilized. Excellent to good outcomes
were recorded in 87% of patients at a mean follow-up of 6 years; 25% of cases
showed some degree of acetabular migration. In order to overcome this comparatively
high complication rate, more vigorous impaction in combination with a stronger mesh
for bone containment was recommended; the utilization of bone chips > 1 cm was
also recommended although not elaborated upon (14,20).
The utilization of cementless cups in combination with impaction grafting in
acetabular reconstruction is not widespread. This is centred on the belief that
impacted allograft bone has no or limited bone ingrowth potential (4-6). Current
dogma further suggests that at least 30-50% contact with host bone is required and
that the stiff, cementless cups with incite bone resorption (stress shielding) through
the disparity in elastic moduli (17).
Trabecular metal _ has a porosity similar to bone and provides an environment more
favorable to bone graft remodeling than conventional metals (7,16,17); it has also
been shown to have excellent ingrowth potential (19,21). The current pilot study has
sought to evaluate the efficacy of TM in combination with impaction grafting of
autograft bone in the repair of a simulated Type III acetabular defect using a sheep
Materials And Methods
The experimental protocol was submitted to, and approved by, the institutional
Animal Welfare Committee.
A unilateral total hip replacement (THR) was performed using a 22 mm (inner
diameter) polyethylene cup and a cemented, modular femoral stem and head using an
already established protocol (22). With the femoral head removed a 2 cm diameter
defect was created in the cranio-dorsal aspect of the left acetabulum. A retainment
mesh was fashioned and secured to the labrum cranial to the acetabulum with 3.5 mm
screws. Milling of the excised femoral head provided cortico-cancellous bone (4 mm
chip size) which was then impacted into the defect created. Subsequently a 1.5 cm
TM plug was placed onto the impacted bone surface; this plug had no contact with the
surrounding host bone (Figure 1).
First generation cementing techniques were then applied to both the polyethylene cup and the femoral stem.
The sheep was recovered in a sling for the first day post-operatively after which
normal weight bearing ensued. Analgesia was provided for three days postoperatively.
At one week post-operatively the sheep was returned to pasture for the
duration of the study.
Evaluative methods employed included radiography, macrophotographs, microradiology
and histology. Radiographs were obtained post-operatively and at 6 and 8
weeks. At 7 weeks the sheep dislocated the reconstruction; this was reduced
successfully under general anaesthesia. The dislocation recurred at 8 weeks
and the sheep was euthanatized. (Figure 2)
Five mm transverse sections were taken spanning the width of the reconstruction and
placed in 10% Buffered Formalum solution. Each section was photographed for a
visual assessment of bone ingrowth. Preserved sections underwent micro-radiography
allowing assessment of bone-implant interface occurrences. Histological assessment
included the administration of a fluorochrome (calcein green) at 6 weeks postoperatively
to enable visualization of new bone formation using fluorescent
microscopy. Unstained ground sections were assessed using fluorescence microscopy
using a Leica DM 6000 B fluorescence microscope attached to the Quantimet Version
550 IW image analysis system. Following fluorochrome analyses, ground sections
were stained with toluidine blue and examined using plain light microscopy on an
Olympus BH-2 microscope (Olympus Optical Company, Japan). Representative
digital microphotographs of the histology were taken.
No untoward effects of the procedure were observed post-operatively with the sheep
released to graze at pasture at one week. The first (calcein green) of three
fluorochrome injections was performed at 6 weeks uneventfully. At 7 weeks the
animal was observed to be lame. Radiographic evaluation under general anaesthesia
revealed a dislocation of the femoral components with the femoral head resting
cranial and dorsal to the acetabulum. This was reduced manually under traction and
the sheep recovered. Normal weight bearing followed. At 8 weeks post-operatively,
lameness was again observed. Radiographs (Figure 2) revealed a recurrence of the
dislocation; the sheep was euthanatized and appropriate specimens retrieved.
Digital macrophotographs of salient sections were recorded and enable some
assessment of the extent of bone ingrowth and/or fibrous tissue formation along the
implant-autograft interface. Sections removed at the periphery of the TM implant
consistently displayed a degree of fibrous tissue invasion at the interface between
implant and autograft bone (Figure 3).
Through the majority of the sections however,
little or no fibrous tissue was observed at the interface (Figure 4).
Figure 5 is a higher magnification view showing cement penetration into the TM and an implant-autograft interface with no fibrous tissue evident.
Microradiographs obtained concur with the findings with macrophotographs. Through
the majority of sections little or no fibrous tissue formation was observed at the
implant-autograft interface (Figure 6).
Assessment of sections under fluorescent microscopy highlighted significant amounts
of new bone formation within the autograft bed, at the implant-bone interface and
within the implant itself (Figure 7,8).
Assessment of the same sections under light
microscopy and stained with Toluidene blue confirmed the presence of new bone
formation at all three locations (autograft, interface, implant). Bone ingrowth into the
outer porous spaces (1-1.5 mm depth) of the TM implant was observed along most of
the surface to varying degrees (Figures 9).
The existing bone stock and type of defect are significant factors in surgical decision
making when undertaking revision of acetabular components. The intent is to restore
anatomy and to achieve stable fixation of the new acetabular component (s). Unlike
more traditional metal options, trabecular metal has a porosity akin to bone and
confers a more favourable mechanical environment in which bone graft materials
(depending on their constitution) can function; this may be osteoconductive,
osteoinductive or combination thereof (7,16,17,19,21). The current pilot study has
provided an opportunity to evaluate the utilization of a Trabecular Metal implant
when used in conjunction with impaction of autologous bone graft material and
cement fixation of components; to this end the model has proven satisfactory.
The literature (5,9) highlights concerns regarding the osseointegration of porous metal
components when pressfit into a bone-deplete acetabulum. Recent studies (7,8) have
evaluated the incorporation of primarily allogenic bone graft material to fill the defect
followed by the placement of a cemented polyethylene cup. The current study has
sought to evaluate the utilization of TM to augment the repair when used in
combination with bone graft material and cemented components. Strong evidence of
the early ingrowth of new bone into a trabecular metal implant has been observed, in
this study, as early as six weeks post-operatively. Microradiography and histological
techniques have shown good continuity of direct bone graft apposition and ingrowth
along the majority of the trabecular metal implant. New bone ingrowth into the TM
was observed extending as much as 1-2 mm . This is observed in the amount of
fluorescent label apparent within the TM (Figure 9) and is corroborated by lightmicroscopy (Figure 7,8).
This augers well for the provision of a stable mechanical
environment in which continued ingrowth and increasing strength of the fixation is
anticipated. The current study was intended to proceed over a three month period with
fluorochromes administered at 6, 8 and 10 weeks. Repeat dislocation necessitated
euthanasia of the animal prematurely; it was believed the dislocation was a response
to a significantly modified polyethylene cup which was quite shallow. Based on the
significant presence of fluorochrome label at six weeks we believe all three labels,
had they been administered, would have been present in the trabecular metal
indicating ongoing new bone ingrowth and complimentary strength of the fixation.
Anticipated future studies will involve the use of an uncemented TM cluster cup in
which migration of the acetabular components will be measured using
radiostereometric analysis in combination with microradiography and histological
This preliminary study has demonstrated that trabecular metal can be successfully
used in impaction grafting in the absence of direct contact with peri-prosthetic host
bone. New bone deposition was observed at depths of 1-3 mm inside the TM
interface, in this study, as early as 6-8 weeks. This augers well for the utilization of
trabecular metal to augment acetabular reconstruction in Type III scenarios. We now
anticipate, that over longer periods of time, with continuing ingrowth and increasing
strength, the provision of a stable mechanical environment will be achieved in
response to the surgical techniques employed in this model.
The authors would like to acknowledge the contributions of Margaret McGee,
Department of Orthopaedics and Trauma, Royal Adelaide Hospital for histological
preparation and Peter Self, Adelaide Microscopy, for microradiography and micro