The hip joint functions effectively as a fulcrum, resulting in a state of
equilibrium between body weight and the opposing hip
abductors5,15,16.
The outcome of this interplay of opposing forces is the ability to maintain a
level pelvis throughout the gait cycle.
The length of the lever arm that acts between the femoral head and the
insertion of the hip abductors (distance "A" in
Fig. 2) is markedly smaller
than that between the femoral head and body weight (distance "B"
in Fig. 2). Therefore, the
abductors must generate a force that is larger than body weight to compensate
for their mechanical disadvantage. Gait analyses and free body diagrams have
shown this discordant biomechanical relationship to translate into
significantly higher (p < 0.05) joint-reaction forces in total hip
replacements without restoration of femoral
offset17.
Conversely, an increase in femoral offset increases the lever arm of the
abductor muscles, thereby reducing the abductor muscle force required for
normal gait. This in turn minimizes the resultant reactive force across the
hip joint and hence results in lower rates of polyethylene wear. Furthermore,
the lateralized position of the femoral shaft relative to the hip center tends
to decrease the prevalence of femoropelvic impingement while concomitantly
improving soft-tissue tensioning (Fig.
3).
In view of these facts, an understanding of the forces that act across the
hip joint provides the surgeon with the capacity to address the factors that
may contribute to inadequate soft-tissue tensioning during total hip
replacement.
The shape and geometry of the proximal part of the femur have been studied
by a number of
investigators6,18.
In a study of fifty consecutive patients scheduled for total hip arthroplasty,
Davey19 made
standard radiographs of the hip, applied templates, and measured femoral
offset after adjustment for magnification. The average offset was 43.9 mm, but
the range was 27 to 57 mm. These results are consistent with others reported
in the
literature4,6,16,20.
A number of factors determine the amount of offset of the femur. Large
femora tend to have more offset than smaller ones. Noble et
al.6 found the
average neck-shaft angle to be 124.7°, with a range of 105.7° to
154.5°. They concluded that hips with a varus neck-shaft angle tend to
have greater femoral offset and hips with a valgus neck-shaft angle tend to
have less femoral offset.
Offset of a femoral component, like offset of the native femur, can be
measured from the center of rotation of the femoral head to the long axis of
the stem. Femoral component offset depends on both the length of the femoral
neck and the neck-shaft angle of the prosthesis.
Reduced offset in a reconstructed hip may result from use of a femoral
component with less offset than was present in the native hip of the patient,
from a valgus position of the femoral stem relative to the femoral shaft, or
from use of a short-necked modular head. Such a decrease in femoral offset
medializes the locus of the abductor muscle insertion, decreases the abductor
moment arm, and therefore increases both the resultant force across the hip
joint and the energy required for normal gait.
Three aspects of the function of a joint replacement are at least
theoretically influenced by femoral offset. These include strength, motion,
and stability.
The increase in the moment created by lengthening of the functional lever
arm (Figs. 4-A and
4-B) increases strength and
therefore lessens the joint reactive forces. This in turn decreases rates of
wear and aseptic loosening, as reported by Sakalkale et
al.13. The
increased abductor strength also decreases the prevalence of a Trendelenburg
gait and enhances stability at the hip joint. The latter concept is supported
by the observation of a sixfold increase in implant dislocation in association
with trochanteric
nonunion21.
Finally, it has been theorized that the displacement of the femur with
reference to the socket and pelvis that occurs with increased femoral offset
increases motion. This lessens the likelihood of impingement and thus provides
a second explanation and basis for enhanced
stability6.
Davey et al.22
investigated the effect of increased femoral offset on the distribution of
strain in the bone and cement mantle while determining the abductor and
resultant forces in a cemented total hip model. Cadaveric femora were
harvested and were tested in a materials testing machine (MTS Systems,
Minneapolis, Minnesota) in a position simulating single-limb stance. Static
vertical loads of 600 N were applied to the femora. Abductor and resultant
forces along with strain in the cement mantle and in the proximal part of the
femur were recorded as offset was increased from 33 to 53 mm. When the femoral
offset was increased by 10 mm, the abduction force decreased by approximately
10% and this was associated with a 10% decrease in force transmission at the
acetabulum. These results concur with Charnley's biomechanical calculations
regarding
offset5.
Manufacturers have considered the implication of precisely restoring
femoral offset when designing femoral implants. An anatomic study of the
proximal part of the femur indicated that if a prosthetic implant system has a
single neck-shaft angle, up to 67% of patients will not have accurate
restoration of the biomechanical center of the hip or femoral
offset23.
Furthermore, it was noted that eight different neck-shaft angles would have to
be available to restore the anatomy accurately in only 50% of patients. The
inference of this finding is that a greater variety of implant sizes might be
necessary in order to restore proper hip balance. Noble et
al.6 called
attention to the need for a greater selection of femoral shaft sizes and
diameters in order to accommodate and restore the neck-shaft angle in a manner
that approximates normal.
All of these factors influence the production and use of total hip
components with regard to implant design, shape, and size variation.
Clinicians should consider these variables when making decisions about implant
selection and surgical technique.
There are five means with which one can increase femoral offset. The first
four are based on altering the geometry of the femoral component or the
proximal femoral anatomy. They include increasing the length of the femoral
neck, decreasing the neck-shaft angle, medializing the femoral neck while
concomitantly increasing femoral neck length, and trochanteric advancement.
The fifth method involves alteration of the geometry of the acetabular liner.
It is clear that, from a clinical perspective, it is important that the
surgeon be able to recognize the implications of the different techniques for
varying offset.
Femoral Component
Increasing Neck Length
Increasing the length of the femoral neck or head increases the resting
length of the hip abductors and, depending on the angle of the femoral neck,
increases their contractile efficiency while concomitantly lengthening the
abductor lever arm. Unfortunately, an increase in the neck length also
increases the limb length, resulting in a limb-length discrepancy. This is an
undesirable clinical outcome in most cases
(Fig. 5).
Decreasing Neck-Shaft Angle
Decreasing the neck-shaft angle reduces the height of the femoral head, and
thus the limb length, while increasing offset. This construct directly
increases the magnitude of the abductor lever arm. It also has the positive
effect of increasing abductor tension, making the muscles more efficient.
However, this change in implant dimension has the negative effect of
increasing the rotational torque imparted to the implant from out-of-plane
forces. The greater varus neck-shaft angle results in an increased torsional
(or out-of-plane) force that tends to rotate the femoral component, especially
with activities involving load transmission during hip flexion and extension
such as stair-climbing. The impact of this change is calculated with the
equation: I= mr2, where I reflects the magnitude of the
out-of-plane rotation; r is the radius, or distance, of the offset; and m is
the mass or applied force. It should be noted that the offset dimension is
squared, thus increasing axial torque at a rate that is greater than the rate
at which abduction strength is enhanced
(Fig. 6).
Medializing the Femoral Neck While Concomitantly Lengthening the
Femoral Neck (Dual or High-Offset Femoral Components)
Dual or high-offset femoral components either vary the neck-shaft angle of
the implant or medialize the neck to vary offset. This geometry maintains the
neck-shaft angle relationship while concomitantly restoring offset. A major
advantage of this technique is that it can be used to enhance abductor
tensioning without substantially affecting limb length
(Fig. 7). Therefore,
medialization and concomitant lengthening of the femoral neck represents the
basis for the dual or high-offset femoral design.
Trochanteric Osteotomy
In this instance, offset is defined as the distance from the center of the
head to the attachment of the abductor muscles or as the perpendicular
distance from the center of the head to the line of action of the abductor
muscles. Since this definition differs from the usual definition of femoral
offset, it may better be termed abductor offset. Trochanteric
osteotomy provides a biomechanical advantage by laterally and distally
advancing the point of insertion of the abductors. It has a positive effect in
that it increases the strength of the abductors and hence decreases the
likelihood of a Trendelenburg gait. The increased mechanical advantage
decreases the resultant moment arm and thus decreases the compressive force
across the joint. This lessens the likelihood of wear and loosening. However,
the procedure does not improve motion or lessen the likelihood of
impingement.
Acetabular Component
Modular "offset" or "lateralized" liners have been
shown to increase offset while preserving limb length. The offset may be
altered by modifying the relationship of the articulation at the socket so
that the center of rotation at the hip is translated both laterally and
inferiorly2. A
laterally displaced socket increases the abductor tension, which is a
desirable outcome. However, it also increases the body weight lever arm
(Fig. 2), which is considered
an adverse outcome. The impact of the latter increase is greater since body
weight acts at a perpendicular distance to the increments of lateral
displacement. On the other hand, improvement of the moment arm depends on the
angle of the line of displacement. In other words, the adverse effect is a 1:1
ratio, whereas the beneficial effect is less than a 1:1 ratio and is
proportional to the line of pull of the abductors. Accordingly, lateralized
liners are typically employed when the surgeon has tried a high-offset femoral
component but additional offset is needed in order to restore the abductor
tension and thereby enhance hip stability
(Fig. 8).
Preoperative Templating
Templating is typically performed on radiographs of the contralateral,
"normal" hip since one of the primary goals of surgery is to
restore normal biomechanical properties to the affected joint. In particular,
when a patient has unilateral disease, the normal joint can be used to measure
the optimal amount of femoral offset that should be reproduced. Three
radiographs—an anteroposterior view of the pelvis, an anteroposterior
view centered at the hip, and a lateral view of the hip joint—are
essential for accurate templating. Furthermore, if there is any evidence of
deformity, previous fracture, or previous surgical interventions, a 3-ft
(0.9-m) standing radiograph from the hip to the ankle joint may be helpful in
terms of surgical planning. When a patient has had a previous
fracturedislocation of the acetabulum, Judet radiographs and/or computed
tomographic images should be made to assess the location and degree of bone
loss3.
Templating permits the surgeon to quantify several important parameters,
including the patient's bone stock, component sizes, anticipated depth of
seating of the femoral component within the canal, potential limb-length
discrepancy, optimal level of proximal femoral resection, and anticipated
position of the acetabular component. Together, these variables will determine
the new center of rotation of the joint. The basic principle of templating is
to reproduce the "normal" anatomic center of rotation and restore
femoral offset while maintaining equal limb lengths.
Limb Length
There are several methods with which limb length can be measured
radiographically. Two of the more common techniques will be described.
The first method consists of drawing a horizontal line through two points
located at the inferior aspect of the ischial tuberosities. Alternatively, a
horizontal line can be drawn between the inferior aspects of the acetabular
teardrops, which may be more reliable points of reference than the ischia. The
teardrop is a more discrete anatomic structure, and therefore its vertical
position is not affected as much by rotation of the
pelvis27. A
vertical line is then extended perpendicularly from the horizontal reference
to the estimated center of each femoral head. The difference in length between
the two vertical lines ("A" — "B") represents an
estimate of the limb-length discrepancy. Alternatively, two lines can be drawn
through the center of the lesser trochanter of each femur and parallel to the
ischial line. The net difference in height between the lesser trochanter and
ischium or femoral head and ischium is then measured. Finally, all
measurements should be reduced by a factor of approximately 20% to account for
the enlargement of the osseous anatomy on the
radiographs28.
Therefore, in this example, increasing the neck length in the affected right
hip by the distance "A" — "B" and then
multiplying this value by 0.80 (to account for the 20% magnification) should
equalize the limb lengths (Fig.
9).
It is important to note that, although radiographs are useful adjuncts for
determinations of limb lengths, radiographic measurements should be adjusted
on the basis of the findings of the relevant clinical examination. For
example, a unilateral adduction contracture will result in a perceived
increase in limb length on the affected side, whereas a fixed flexion
contracture tends to result in an overestimation of any shortening that may be
present. Furthermore, patients with fixed pelvic obliquity tend to have
overcorrection or undercorrection as a result of an alteration in the relative
positioning of the osseous landmarks used for templating and determinations of
limb lengths. Accordingly, one of the most important questions that the
clinician should ask the patient is what is his or her perceived limb-length
discrepancy (if
any)3.
Acetabular Component
Templating typically is begun on the acetabular side of the joint with the
more normal hip used as a reference. The orientation of the acetabular shell
is typically 45° relative to the horizontal plane (on the anteroposterior
radiograph) and in approximately 20° of anteversion (on the lateral
radiograph). The apex of the acetabular component should be positioned just
lateral to the teardrop.
An appropriately sized acetabular component should be covered at its
superolateral margin by host bone with avoidance of excessive overhang or
underhang. Finally, if the acetabular component is to be fixed with cement,
one should allow a minimum of 2 mm between the acetabular template and the
host bone to allow for an adequate cement mantle. The template that satisfies
all of these criteria is then selected, positioned, and marked at its center,
which will represent the new center of rotation for the joint.
Femoral Component
After the center of rotation of the acetabular component has been
established, the femoral template is superimposed on the radiograph. An
anteroposterior radiograph with the femur internally rotated approximately
20° (so that the true neck-shaft angle is in the same plane as the
radiograph) provides the surgeon with the most representative view of the
proximal femoral anatomy. The optimal component size is then established from
the radiograph by matching the geometry of the implant to that of the host
bone. The various implant designs will influence the type and size of the
components that are selected. For example, for cemented femoral prostheses, a
minimum of 2 to 3 mm of cement mantle is required to provide adequate
fixation, whereas, for proximally coated implants, metaphyseal fit and fill
are of greatest importance. Moreover, the manufacturers of extensively
porous-coated prostheses advocate a minimum of 4 to 5 cm of cortical
interdigitation or "scratch fit" to obtain adequate torsional
stability and minimize subsidence.
Once the appropriate type and size of femoral component have been
determined, the template should be positioned so that it is parallel to the
anatomic axis of the proximal part of the femur, with particular care taken to
avoid varus or valgus malalignment. If no limb-length discrepancy is present,
the surgeon should align the center of the appropriate femoral head template
with the anticipated center of rotation previously marked on the radiograph.
However, if the affected hip is short, then the head center should be
positioned above the anticipated center of rotation by a distance that is
equal to the measured limb-length discrepancy ("A" —
"B"). Lastly, the neck length is marked and measured relative to
its distance above the lesser trochanter. The optimal neck length can then be
determined intraoperatively by testing various head lengths. If the center of
the trial femoral head is positioned medial to the planned center of rotation,
femoral offset will necessarily be increased and the joint reactive forces
acting at the hip will be correspondingly reduced. Conversely, if the femoral
head center lies lateral to the center of rotation, offset will be reduced,
resulting in lower abductor strength and increased joint reaction force.
Clearly, this latter scenario should be avoided whenever
possible3,29.
Intraoperative Measurements
Limb Length and Femoral Offset
In order to equalize limb lengths and restore offset, the surgeon should
first measure the limb length and femoral offset on the affected side prior to
dislocation and again with the trial implants in place. A jig that measures
both of these parameters depends on a fixed reference point. The proximal
reference consists of a Steinmann pin placed into the tubercle of the iliac
crest through a percutaneous stab wound. A second point is then marked on the
lateral aspect of the greater trochanter. With the hip in full extension, limb
length and offset can then be precisely measured
(Fig. 10) and may be adjusted
as required8.
As discussed previously, there are four means with which femoral offset can
be effectively restored intraoperatively. Of these four, only the medialized
high-offset femoral component design was found to not appreciably alter limb
length. Accordingly, in the absence of a limb-length discrepancy, these two
techniques are our preferred system for restoring femoral offset.
Special Tests
In addition to modifying the geometry of the femoral or acetabular
component, preoperative templating, and intraoperative measurement of offset
and limb length, there are several intraoperative maneuvers that can be
employed to assess both soft-tissue tensioning and limb lengths. Typically,
all of these techniques are performed with the trial components. This affords
the surgeon the flexibility to adjust length or offset by using various
combinations of sizes and offset designs to obtain an optimal clinical result.
Specifically, these maneuvers consist of the shuck test, dropkick test,
leg-to-leg comparison, and additional stability tests.
Shuck Test
The shuck test facilitates an assessment of stability by distracting the
hip joint through the application of in-line traction in a distal direction.
This maneuver allows a subjective determination of the overall soft-tissue
tension around the hip joint. By testing various combinations of neck offsets
(high or standard), neck lengths, and possibly liners (standard or
lateralized), the surgeon can assess which trial components provide optimal
tensioning of the soft-tissue
structures7.
Dropkick Test
The dropkick test is a maneuver whereby the hip is held in extension while
the knee is concomitantly flexed to 90°. If the extremity has been
overlengthened, the extensor mechanism becomes excessively taut and this may
manifest itself as a tendency for the knee to passively swing into extension
when the leg is released (Fig.
11).
Leg-to-Leg Comparison
During patient positioning, it is essential that the patient's
contralateral heel and knee are palpable through the drapes so that a
side-to-side comparison of the treated and untreated limbs can be performed
both prior and subsequent to insertion of the trial components. This technique
serves as yet another means of assessing and comparing limb lengths in the
operating room.
Additional Tests
Additional tests include an assessment of stability both in extension with
concomitant maximal external rotation
(Fig. 12) and in 90° of
flexion of the hip and knee with concomitant maximal internal rotation
(Fig.
13)7.
It is important to note that, under all circumstances, the establishment of
hip stability must take precedence over equalization of limb lengths and
restoration of femoral offset. Accordingly, as a component of obtaining
informed consent, it is imperative that the surgeon discuss the potential of
limb-length discrepancy with the patient.