A Combined Anteversion Technique for Robotic Arm Guided Total ...

A Combined Anteversion Technique
for Robotic Arm Guided Total Hip Arthroplasty
Author: Lawrence D. Dorr, M.D., Medical Director, Total Joint Reconstruction,
Dorr Arthritis Institute, Good Samaritan Hospital
Three primary goals in performing total hip arthroplasty
(THA) are achieving fixation of the implants, using durable
articulating materials, and preventing impingement. Through
the history of cemented THA the technical maneuver to
mate the femoral and acetabular implants correctly has been
to strive for “normal position”, i.e., the femoral stem at 15 o
anteversion and the cup in a safe zone of 15 o ± 10 o . The
advent of cementless femoral fixation changed this principle
of component placement. We learned that a cementless
stem can only be implanted within 10 o -20 o anteversion in
less than 50% of patients. 1,2 In fact, two studies showed we
often didn’t get cemented implants into that range of
anteversion. 1,3 So cementless implants caused us to
change the paradigm of the THA operation. We needed to
know the anteversion of the femoral stem before we
implanted the acetabular cup since we could not adjust the
cementless femoral stem to change its anteversion.
Combined anteversion had to be obtained by adjustment
with the cup. To be certain the hip replacement was in the
safe zone to prevent impingement and promote stability, the
femur must be prepared first and then the cup anteversion
is increased or decreased according to the achieved stem
anteversion. Ranawat termed this concept of mating the
implants “combined anteversion.”
With this paradigm shift of combined anteversion has come
a second change in thinking: cup inclination and anteversion
are determined on the body’s functional plane, not just the
anatomy of the acetabulum within the pelvis. The functional
plane is the true coronal plane and is important because the
pelvis tilts forward and backward as a patient stands and
sits (up to 30 o change in pelvic position). A cup placed on
the functional plane better covers the femoral head as the
femur internally rotates with sitting and externally rotates
with standing. The functional plane for cup anteversion is
calculated by measuring the tilt of the pelvis on a CT scan.
Cup anteversion is changed 0.8 o for each 1 o of tilt (10 o tilt
means functional cup anteversion is 8 o more than the
anatomic anteversion with posterior pelvic tilt and 8 o less
anteversion with anterior pelvic tilt).
Defining the stem and cup combined anteversion safe
zone has been done clinically by Ranawat, 4 with computer
modeling by Widmer and Zurfluh, 5 and using computer
navigation and CT scans by Dorr et al. 6 The safe zone is
25°-45°. Men are commonly in the range of 25°-35° and
women 30°-45° (50 o is safe for flexible women). The mean
combined anteversion found for both Widmer and Zurfluh 5
in their computer study and Dorr et al., 6 in a clinical study
was 37 o . This corresponds to the human newborn cadaver
study of combined anteversion by McKibben who first
defined normal in 1970 as 30 o -40 o . 7
Combined anteversion is important because it is the
surgical technique which gives the greatest protection
against impingement of the components, and together
with correct leg length and offset provides protection
against bone-on-bone impingement. The number one
cause of failure in THA is impingement because it causes
dislocation, loosening of implants, accelerated wear and
pain. 8 It has become a critical factor in longevity because
we are treating THA patients who are younger and more
active and who use much greater range of motion with their
post-operative activities. Retrieval studies show a high
percentage of impingement, especially near the 10 o’clock
position of the cup which means it is occurring with
extension of the hip (normal gait) even more than with
flexion (the most common position of dislocation). This fact
emphasizes the importance of avoiding impingement for
comfort with day-to-day activities and reducing both wear
and loosening with these activities.
Combined anteversion is the only method that gives
predictable, reproducible results with cementless implants.
Preparing and implanting the cup first, especially if screws
are used (or the cup is cemented), locks the surgeon into
a cup position that may be 10 o -20 o from optimal once the
cementless femoral stem is implanted. This traditional
sequence of THA reduces the surgeon’s options at surgery,
which should be avoided.
Precision with achieving correct combined anteversion
has been exponentially improved by the use of robotic
technology in the operating room. In previous studies, the
precision of an experienced surgeon judging anteversion
of the stem was 8 o and of the cup was 11 o (precision
means the magnitude of error possible). 2,9 Surgeons have
outliers of the cup of 10 o or more in 10%-16% of cases
(depending on experience). 9 In another study, the cup
was outside the planned safe zone 53% of the time. 10

2
A Combined Anteversion Technique
for Robotic Arm Guided Total Hip Arthroplasty
Precision with navigation was 5%. 9 The advantage of the
robotic arm is the mechanism which ensures the surgeon
reams the acetabulum to plan. Reproducibility with this
precision and accuracy of cup implantation achieved with
the robotic arm is not possible by the most experienced hip
surgeon using just their judgment, experience, and instinct.
THA Combined Anteversion Technique
1. Pre-operative planning of the cup on the CT
scan determines the cup size and position,
inclination, anteversion, and center of rotation
Cup inclination and anteversion is referenced on the
functional plane. Do not oversize the cup because its
metal edges will protrude beyond the bony rim of the
acetabulum. Do not “bury” the cup, or the bony rim
of the acetabulum will cause impingement (the center
of rotation, COR, will be too superior or medial in this
situation). Inclination should be 40 o because the margin
of error of the robotic arm is 5 o degrees and inclination
should never exceed 45 o , beyond which has been
shown to increase wear and the rate of dislocation.
This occurs because when a patient sits, the cup
inclination increases 25 o from its standing or supine
inclination, therefore 50 o of inclination at surgery means
the cup would be 75 o inclined at sitting. Dislocation can
occur with sitting by the femoral head dropping out
of a cup inclined 75 o to 80 o . This risk is compounded
if the hip is also short by 3mm or more. This cause of
dislocation explains why some patients say they
dislocated while sitting when they simply turned
to their left or right.
Figure 1: The combined anteversion page in the RIO ® THA software allows
the surgeon to adjust the acetabular cup anteversion based on the already
determined femoral anteversion.
position. Estimating anteversion from just the cut surface
of the neck is not satisfactory because anteversion is
determined by the neck angle, the isthmus of the true
calcar, and the diaphyseal bow. Pre-operative planning
of the correct level of the neck cut is done on the CT
scan with the stem in place. Because the cup has to be
medialized to achieve coverage, it is preferable, when
possible, to plan the neck cut to position the center of
the femoral head medial to the femoral head COR to
increase the femoral offset (compensates for decreased
cup offset) (Figure 2).
The anteversion pre-operative plan is initially set at
15 o -20 o (surgeon choice) and the surgeon has the option
to adjust the cup anteversion after the femoral broach
version is determined. Once the femoral stem anteversion
is known, a combined anteversion page will be displayed
on the screen and this page also provides suggested
gender-specific clinical ranges 6 (Figure 1).
2. Pre-operative planning of the femur
The version of the native bone is known from the CT
scan, but this does not tell the surgeon the anteversion
of a stem inserted into the femur— which can differ by
10 o from the bony version. Anatomic stems and blade
stems can increase anteversion; Zweymuller-type stems
decrease anteversion as can stems with large proximal
bulk. So the anteversion of the stem must be determined
intra-operatively by observing or measuring the broach
Figure 2: The stem planning page in the RIO ® THA software allows the
surgeon to position the center of the femoral head medial to the natural
femoral head COR to increase femoral offset if desired.

3
A Combined Anteversion Technique
for Robotic Arm Guided Total Hip Arthroplasty
3. Intra-operative preparation and measurements of
the femur
After dislocation of the hip, placement of the femoral
array and registration of the femur into the software
are completed:
a) Neck resection– The neck cut is marked on the bone
and the level of cut confirmed after head removal.
b) Stem anteversion– When broaching is complete the
broach array is attached to the neck of the broach and
the value of anteversion is measured.
c) Hip length and combined offset– The surgeon should
observe whether the broach is seated to the stageline
transition zone from broach teeth/porous coating to
smooth metal or is above or below that level. The
broach (stem) level can change the modular head
length chosen. The anticipated change in leg length
and offset is displayed on the screen when the broach
array is attached to the final broach (Figure 3). Because
the COR of the cup is fixed by the pre-operative plan,
these numbers should confirm the pre-operative plan
for change in hip length and offset unless the broach
is left proud or sunk deeper than planned.
Figure 3: The femoral broach version is captured based on the patient’s
native anatomy before proceeding to the combined anteversion page.
4. Intra-operative cup position
Cup inclination of 40 o is a fixed value and should
not change. With the broach anteversion known,
anteversion of the cup of 15 o -20 o can now be changed
to provide combined anteversion of 25 o -35 o in men
and above 30 o in women. Both men and women with
a retroverted hip will have combined anteversion of
25 o -35 o . The higher the combined anteversion, the more
favorable. But many arthritic hips have retroversion or
very low anteversion of the bony hip, thus the combined
anteversion is low. This is more common in men.
To more easily accomplish cup anteversion of 20 o
or more, the operating table (patient) can be tilted
posteriorly 20 o . This allows the robotic arm to be directed
straight into the acetabulum and negates the femur,
in its anteriorly displaced position, from blocking the
achievement of anteversion with reaming and the cup.
Technical Tips for Combined
Anteversion
1. Use of the MAKOplasty ® THA application with the
robotic arm tactile constraint for reaming and cup
implantation greatly facilitates reproducible achievement
of the desired combined anteversion. This program
provides a precision of 5 o for inclination and anteversion
and 2mm for cup COR.
2. Prepare the femur first so the anteversion of the femur
is known. The femoral anteversion is the fixed value. Cup
anteversion can be adjusted to achieve the desired
combined anteversion. If the femoral stem is retroverted:
a) Rongeur the posterior-lateral corner of the femoral
neck and attempt to increase stem anteversion
by broaching with a smaller size and sequentially
increasing size until a secure press-fit of the broach is
obtained. If the broach tends to return to its retroverted
position, it means no change in anteversion can be
accomplished (to persist could fracture the femur).
b) Use a modular stem to increase anteversion.
c) Cement a stem. Do not antevert the cemented stem
more than 5 o -10 o because anteversion of 15 o -20 o
degrees will result in in-toeing by the patient (because
anteversion is changed too much from their normal).
3. If femoral stem can be placed in varus (medial to the
femur’s natural COR), this is advantageous because the
cup COR is always medialized. Medialization of the cup
decreases combined offset, whereas a medial femoral
stem COR increases combined offset so that the two
balance each other.
4. In a hip with a retroverted acetabulum (pincer
impingement), the posterior acetabular wall is low and
cup coverage may be limited to 20 o anteversion. If this
is the situation, and more cup anteversion is needed for

4
A Combined Anteversion Technique
for Robotic Arm Guided Total Hip Arthroplasty
a combined anteversion of 30 o , use a hooded polyethylene
liner to avoid metal protruding above the posterior
acetabular wall (however, posterior-superior protrusion
of metal of 3mm is acceptable). In these hips, remember
to remove the anterior osteophyte, and/or prominent
anterior wall caused by retroverted acetabular rotation,
to prevent impingement.
5. When the biomechanical reconstruction is complete,
the final MAKOplasty ® THA application screen displays
two columns, the preoperative plan and the achieved
final value of cup inclination, stem anteversion, cup
anteversion, combined anteversion, hip length, and
combined offset (Figure 4). The surgeon should confirm
Figure 4: The reduction results page in the RIO ® THA software displays the
pre-operative plan and the achieved final values of all measured parameters.
combined anteversion, hip length, and combined offset
manually by maneuvering the leg through a complete
range of motion. This is to ensure that in extensionexternal
rotation, the lesser trochanter does not impinge
on the ischium and at the acetabulum, the metal neck
does not impinge on the posterior rim of the cup or
bone. This is also to verify that with external rotation
and abduction, the posterior trochanter does not
impinge on the posterior acetabulum. Also one
fingerbreadth clearance should be present between the
greater trochanter and posterior acetabulum with this
position. In full flexion, the femoral head should not
drop out inferiorly. In 90 o of flexion, adduction and
internal rotation to full soft tissue tension, there should
be no subluxation or dislocation posteriorly. If dislocation
occurs in this position, use a hooded polyethylene liner
with the apex of the hood at 8 o’clock for the right hip
and 4 o’clock for the left hip.
References
1. Wines AP, McNicol D. Computed tomography
measurement of the accuracy of component version in
total hip arthroplasty. J Arthroplasty. 2006;21(5):696-701.
2. Dorr LD, Wan Z, Malik A, Zhu J, Dastane M, Deshmane
P. A comparison of surgeon estimation and computed
tomographic measurement of femoral component
anteversion in cementless total hip arthroplasty. J Bone
Joint Surg Am. 2009;91(11):2598-2604.
3. Pierchon F, Pasquier G, Cotton A, Fontaine C, Clarisse J,
Duquennoy A. Causes of dislocation of total hip
arthroplasty: CT study of component alignment. J Bone
Joint Surg Br. 1994;76(1):45-8.
4. Ranawat CS, Maynard MJ. Modern techniques of
cemented total hip arthroplasty. Tech Orthopedics.
1991;6:17-25.
5. Widmer KH, Zurfluh B. Compliant positioning of total
hip components for optimal range of motion. J Orthop
Res. 2004;22(4):815-21.
6. Dorr LD, Malik A, Dastane M, Wan Z. Combined
anteversion technique for total hip arthroplasty. Clin
Orthop Relat Res. January2009;467(1):119-127.
7. McKibbin B. Anatomical factors in the stability of the
hip joint in the newborn. J Bone Joint Surg Br. 1970;
52:148-59.
8. Malik A, Maheshwari A, Dorr LD. Impingement with
total hip replacement. J Bone Joint Surg Am. 2007;
89(8):1832-42.
9. Dorr LD, Malik A, Wan Z, Long WT, Harris M. Precision
and bias of imageless computer navigation and surgeon
estimates for acetabular component position. Clin Orthop
Relat Res. 2007;465:92-9.
10. Callanan MC, Jarrett B, Bragdon CR, Zurakowski D,
Rubash HE, Freiberg AA, Machau H. The John Charnley
Award: Risk factors for cup malpositioning: quality
improvement through a joint registry at a tertiary
hospital. Clin Orthop Relat Res. 2010;469(2):319-29.
2555 Davie Road | Fort Lauderdale, FL 33317 | 866.647.6256 | makosurgical.com
© MAKO Surgical Corp. 206709 r00 02/12