Abstract
Background: In patients with symptomatic hip impingement, surgical
resection of the femoral head-neck junction may improve the range of motion
and relieve pain. A risk of this procedure is fracture. We evaluated the
amount of resection of the anterolateral aspect of the femoral head-neck
junction that can be done safely.
Methods: Cadaveric proximal femoral specimens (fifteen matched
pairs) were divided into three groups: 10%, 30%, or 50% of the diameter of one
femoral neck was removed, and the contralateral femoral neck was left intact
to serve as the control. A compressive load was applied directly to the
femoral head. Peak load, stiffness, and energy to fracture were compared among
the groups.
Results: The energy to fracture differed significantly (p = 0.0015)
among the 10%, 30%, and 50% resection groups. The peak load after the 50%
resection was significantly less (p = 0.0025) than that after the 10% or 30%
resection. With the numbers available, there was no significant difference in
peak load between the 10% and 30% resections.
Conclusions: Resection of up to 30% of the anterolateral quadrant of
the head-neck junction did not significantly alter the load-bearing capacity
of the proximal part of the femur. However, a 30% resection significantly
decreased the amount of energy required to produce a fracture. Thirty percent
should be considered to be the greatest feasible amount of resection because
of the change in the pattern of the femoral head-neck response to axial loads
that we observed.
Hip impingement is a diagnosis that has been increasingly recognized
among young patients with hip
pain1-23.
Two different types of impingement have been
described1-5.
Overcoverage impingement, or a "pincer" effect, occurs between the
anterior wall or labrum of the acetabulum and the femoral head. This is
typically due to a decrease in anteversion of the acetabulum or overcoverage
of the femoral head (coxa profunda or
protrusio)6. A
so-called cam-effect impingement occurs when the femoral head-neck junction
has an abnormally large radius resulting in insufficient offset. Widening of
the femoral neck reduces its concavity, creating an impingement over the
acetabular rim. Thus, the anterolateral junction is forced under the
acetabular rim, resulting in labral injury and deterioration of the
cartilage1,3,7-12.
Options for treatment of impingement include nonoperative management
(nonsteroidal anti-inflammatory drugs and avoiding positions of impingement),
arthroscopic débridement, trimming of the anterior aspect of the
acetabular rim after surgical dislocation of the hip, periacetabular osteotomy
when impingement is secondary to an acetabular torsion abnormality, and
surgical resection of a femoral neck bump and/or part of the anterolateral
aspect of the neck when the primary anatomic abnormality is secondary to
insufficient head-neck
offset2,3,5,10,18,19.
Resection of a portion of the anterolateral aspect of the femoral head-neck
junction improves the femoral head-neck ratio, increasing the range of motion
before impingement
occurs19. However,
a risk of this procedure is a femoral neck fracture.
We are unaware of any studies documenting how much resection of the
anterolateral aspect of the femoral head-neck junction is possible before the
structural integrity of the femoral neck is markedly compromised. The purpose
of this study was to evaluate the stiffness, amount of energy absorbed, and
peak load to failure after progressive partial resection of the anterolateral
aspect of the femoral head-neck junction for the correction of cam-type
impingement.
Fifteen pairs of femora were harvested from fresh-frozen cadavers at
the Department of Anatomy of our institution. The mean age at the time of
death of the specimen donors was seventy-nine years (range, fifty-seven to
ninety-eight years). The mean height was 167 cm (range, 152 to 185 cm), and
the mean weight was 62 kg (range, 44 to 83 kg). The cadavers were thawed at
room temperature. The proximal parts of both femora were isolated and cleaned
of soft tissues, and the circumferences of the femoral head-neck junctions
were measured (Fig. 1). The
femora (fifteen matched pairs) were divided into three groups of five pairs
each. With this matched-pair study design, the contralateral, control femur
was of the same age as the experimental femur and, we assumed, had a similar
bone density. One side (right or left) was randomly assigned to the
experimental group and the other, to the control group.
The circumference of the head-neck junction was divided into four
quadrants: anterolateral, anteromedial, posterolateral, and posteromedial. The
anterolateral quadrant was resected in each case, and the length of the
resection always encompassed the full anterolateral quadrant of the femoral
head-neck junction (25% of the circumference perimeter). The depth of the bone
resection was calculated as a percentage of the head-neck diameter, as
previously measured, of each femur (Fig.
1); 10%, 30%, or 50% of the head-neck diameter was resected
(Fig. 1), with the percentage
of the resection referring to the depth of the osteotomy.
Figure 1, D shows a
completed wedge resection. The base width of the wedge resection at the
anterolateral quadrant of the head-neck junction was 15 mm in all cases. The
matched controls were tested intact.
All of the resections were performed with use of 5-mm-diameter and
3-mm-diameter round burrs in order to specifically match the resection
dimensions desired.
The offset of the femoral head-neck junction was improved by increasing the
percentage of the bone resected with the wedge osteotomy. This, in turn,
increased the range of motion of the hip before impingement (Figs.
2-A,
2-B, and
2-C).
The thirty proximal femoral specimens were then potted in
methylmethacrylate and clamped in a servohydraulic materials testing system
(MTS, Minneapolis, Minnesota); the specimens were secured in a vise in 25°
of adduction in the coronal plane and neutral in the sagittal
plane20. The
materials testing system was used to apply compressive loads directly to the
femoral head with a flat applicator (Fig.
3). The axial loading protocol has been used previously to test
devices for fixation of femoral neck
fractures20.
Compressive loading was applied under displacement control at a rate of 20
mm/min until a femoral fracture occurred.
Statistical Analysis
With five pairs of specimens in each group, there was an 80% power to
detect a difference in mean ratios of two standard deviations and a 50% power
to detect a difference of 1.4 standard deviations. According to
Cohen21, this is a
large effect size.
Force and displacement data were recorded for each specimen at a sampling
frequency of 60 Hz. The fracture was identified as a sharp drop of the force
value in the force-displacement graph. The slope of the linear region of the
force-displacement curve represented the stiffness of the proximal part of the
femur and was assessed with use of linear regression. The forces achieved at
the time of the fracture of the femur represent the peak load. The energy to
fracture was calculated as the area under the force-displacement curve, from
the start of the test until the point of peak load.
The stiffness of the femoral neck, the peak load before fracture, and the
energy to fracture were compared among the three groups (10%, 30%, and 50%
resection). The change in the measured variables compared with those for the
control were calculated as (case - control)/control.
Once a normal distribution of the data was confirmed, a
Student-Newman-Keuls post hoc test was performed to determine whether a
significant difference in energy, stiffness, or load-bearing capacity was
associated with the percentage of resection. A p value of <0.05 was
considered to be significant.
The peak load, stiffness, and energy to fracture for the treated
femora (after resection of 10%, 30%, or 50% of the anterolateral aspect of the
femoral head-neck junction) and the controls are shown in
Table I.
In the 10% resection group, all of the fractures observed in the treated
and control specimens were either at the trochanteric area or a splitting of
the femoral head (away from the head-neck junction). There were no significant
differences in stiffness, peak load, or energy to fracture between the
resected and control specimens in this group (p > 0.05).
In the 30% resection group, all of the treated femora fractured at the
level of the bone resection. The peak load following the resections was about
85% of the peak load in the intact specimens. All of the control specimens
fractured either at the trochanteric area or in the femoral head (a splitting
fracture), away from the head-neck junction.
In the 50% resection group, all of the specimens with the resection
fractured at the level of the resection. Three of the five treated femora
fractured at approximately 1200 N of axial load (a load approximately equal to
the forces applied to the hip in a person of average weight during single-limb
stance20,22).
The fractures in the control specimens occurred away from the head-neck
junction, either at the trochanteric area or in the femoral head (a splitting
fracture).
The 10% and 30% resections resulted in less than a 5% change in stiffness
compared with the controls. The 50% resections resulted in a mean decrease in
stiffness of 13%. With the numbers available, the differences in the stiffness
values measured in the 10%, 30%, and 50% resection groups were not significant
(p = 0.5) (Fig. 4-A).
The 10%, 30%, and 50% resection groups required progressively less energy
before the peak load was achieved. These differences were significant (p =
0.0015) (Fig. 4-B). Compared
with the controls, the 30% and 50% resections produced 20% and 66% reductions,
respectively, in the mean amount of energy absorbed before the peak load.
The 10% resection resulted in less than a 1% change in the mean peak load
compared with that in the control. In contrast, the 30% resection resulted in
a 16% reduction in the mean peak load, and the 50% resection reduced the mean
peak load by 43%. The specimens with the 50% resection carried significantly
less load than the other two groups (p = 0.0025). With the numbers available,
there was no significant difference in the mean load-carrying capacity between
the 10% and 30% resection groups (Fig.
4-C).
Resection osteoplasty of the femoral neck has been previously
described for correction of malunited femoral neck fractures and for poor
head-neck
offset3,5,10,19.
Ganz et al.5
described surgical dislocation of the adult hip as a technique for achieving
full access to the femoral head and the acetabulum. They reported no cases of
osteonecrosis in 213 hips treated with this method. One hundred and sixty-four
of the hips were operated on because of anterior impingement resulting from an
idiopathic nonspherical femoral head, mild slipped capital femoral epiphysis,
or poor offset at the head-neck junction. Eijer et
al.10 recently
described improvement of the anterior head-neck offset to treat posttraumatic
impingement.
Treatment goals in this patient group should be pain relief and, hopefully,
prevention of further damage to the cartilage and subsequent osteoarthrosis.
Surgeons using this technique need to know the amount of bone that can be
removed safely before catastrophic weakening of the femoral neck occurs. In
this study, we attempted to answer that question. However, our study does have
some limitations. The average age of the donors of the specimens was greater
than the typical age of a patient being treated with joint-preserving
surgery2,5,8,10,13,15,23,24.
Our samples were probably more osteoporotic than the bone of younger patients,
and this would decrease their loading capacity. Thus, any amount of bone
resection that was found to be safe in our specimens would probably be safer
in a younger patient. Furthermore, we used the contralateral limb as a control
so we are reporting relative weakening.
Bergmann et
al.22 described the
functional loads supported by the hip during normal activities. They reported
that 1500 N of load, corresponding to 190% of the individual's body weight,
was supported by the hip when an individual was standing; 1300 N, or 156% of
the individual's body weight, was supported by the hip when the individual was
sitting; and 2000 N, or 242% of the individual's body weight, was supported by
the hip when the individual was walking slowly. In our study, resection of 50%
of the diameter of the head-neck junction weakened the bone significantly.
Three of the five specimens with this amount of resection fractured when
approximately 1200 N of axial compression force was applied. Therefore, from a
clinical viewpoint, this amount of resection can lead to a fracture even under
physiological loads on the hip.
None of the specimens with the 30% resection fractured under <3000 N of
axial compression, despite the age of the specimen donors. Even though the
energy required for a fracture was significantly lower in the 30% resection
group (a decrease of 20% compared with its controls) than it was in the 10%
resection group, the peak load capacity did not differ significantly between
the 10% and 30% resection groups. A 30% resection appeared to create a new
weak point at the level of the osteotomy that changed the way that the
head-neck junction reacted when it was subjected to a detrimental axial load.
The fractures in the 30% resection group occurred at the level of the
osteotomy; however, the axial load supported before the peak load (fracture)
did not differ significantly from that of the controls. Therefore, a fracture
can occur after a 30% resection and, if it does, it probably will occur at the
level of the osteotomy and require less energy than is required for a fracture
of a normal hip.
These data support the conclusion that up to 30% of the anterolateral
quadrant of the head-neck junction can be resected without significantly
altering the load-bearing capacity of the proximal part of the femur. However,
a 30% resection probably represents the upper limit of resection, and this
amount of resection significantly decreases the energy to fracture.
Furthermore, this amount of resection results in a change in the failure
pattern of the femoral head-neck junction under axial loading.
In this study, the 10%, 30%, and 50% resections referred to the depth of
the osteotomies. If the diameter of a femoral head-neck junction is 50 mm
intraoperatively, a 15-mm-deep osteotomy (30% resection) in the anterolateral
quadrant should be the very highest limit of resection. Clinically, the senior
author (R.T.T.) has noted that usually, in order to achieve a goal of 110°
to 115° of flexion without further impingement, resection of approximately
20% of the head-neck-junction diameter (a 10-mm-deep osteotomy in the above
example) is enough. However, a greater resection may be necessary in some
instances (for example, in a patient with a major lack of offset of a small
femoral head). As these data were obtained from cadavers, one can assume that
the bone of young patients being operated on would be stronger and more
resistant to fracture. On the basis of our results, we propose that no more
than 30% of the diameter of the femoral head-neck junction should be resected
to treat hip impingement even if the desired range of motion has not been
achieved. ?
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