Is Current TKA Designed to Restore Normal Trochlear Groove Anatomy?
+1,2Varadarajan, K M; 1Yue, B; 1Freiberg, A A; 1Rubash, H E; 1Li, G
+1Bioengineering Laboratory, Department of Orthopaedic Surgery, Massachusetts General Hospital/Harvard Medical School, Boston, MA, 2Department of
Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA
Senior author gli1@partners.org
DISCUSSION:
This study presented a detailed comparison of the three-dimensional
geometry of the trochlear groove in normal and TKA knees. The results
showed that for the particular implant designs investigated in this study
(NexGen CR and LPS, Zimmer Inc.); (1) external rotation of the
Proximal point
TKA
Component
A
Bone
Cartilage
θ
B
TEA
C
θmax
Distal point
Figure 1. Planes rotated about TEA used to section cartilage of
normal knee and surface of TKA. Point B represents first section at
which normal trochlear sulcus becomes discernable.
Trochlear
bisector
angle
Sulcus
Lateral
Condyle
Height
Medial
Condyle
Height
Sulcus
Height
Cartilage / TKA
Surface
TEA
midpoint
TEA
Sulcus ML
Location
Figure 2. Trochlear groove geometric parameters measured. AP =
anteroposterior, ML = mediolateral.
6
≈ 0°
≈ 30°
Medial
RESULTS:
Significant differences were noted between sulcus location of normal
knees and knees with externally rotated TKA (Fig. 3). Proximally,
between θo (θ/θmax %) = 43.5% and 58.7%, the TKA sulcus was more
lateral (difference 0.6 – 3.5 mm, p < 0.034), and distally for θo = 73.9%
to 83.1% the TKA sulcus was more medial (0.7 – 0.9 mm, p < 0.047).
Failure to externally rotate the component shifted the sulcus medially,
making the difference (0.5 – 2.0 mm) apparent over a larger proportion
of the trochlea. The LPS and CR components had identical trochlear
groove geometries. Therefore Fig 3 shows data only for CR TKA.
The normal knees and knees with externally rotated TKA also
showed significant differences in trochlear bisector angle. Proximally for
θo = 43.5% to 56.5%, the TKA knees showed smaller values of bisector
angle (0.8 – 4.4°, p < 0.018), and distally between θo = 93.5% and 100%,
the TKA knees showed larger values (1.1 – 2.5°, p < 0.034). Failure to
externally rotate the TKA reduced differences proximally, but increased
differences over the central and distal portions.
Significant differences were also noted between condyle heights of
normal knees and knees with externally rotated TKA. Proximally
between θo = 43.5% and 82.6%, the TKA knees had smaller lateral
condyle height (0.8 – 5.9 mm, p < 0.013), and distally from θo = 95.5%
to 100% the TKA knees had larger lateral condyle height (0.9 – 1.1 mm,
p < 0.003). Additionally, between θo = 43.5% and 73.9% the TKA knees
had smaller medial condyle height (0.8 – 2.5 mm, p < 0.012). Failure to
externally rotate the TKA further reduced the medial condyle height.
Cutting
plane
4
CR TKA (3 deg)
*
CR TKA (0 deg)
Cartilage
2
≈ 105°
0
-2
-4
Lateral
METHODS:
The normal trochlear geometry was first measured using 3D models
of femurs created from MRI scans of healthy subjects’ knees (12 male,
11 female, avg age 30.9 ± 9.0 yrs). Next, the trochlear geometry was remeasured following a virtual TKA procedure performed to mount
femoral TKA components on to the knee models in accordance with
standard surgical protocol, including 3° external rotation relative to the
posterior femoral condyle (NexGen CR and LPS, Zimmer Inc).
Additionally, the trochlear anatomy was measured with the component
aligned with the posterior condyle (no external rotation). Finally, the
trochlear anatomy before and after the simulated implantations were
compared using paired t-test, with significance level set at p <0.05.
To measure the trochlear geometry, a custom computer program was
used to create planes through the distal femur spanning the most
proximal point on the femoral TKA component (point A, Fig. 1), to the
most distal point on the intercondylar notch (point C, Fig. 2). The planes
were rotated about the transepicondylar axis (TEA) in ~2.3º increments
to create 47 cross-sections through the femoral cartilage and the TKA
component. Finally these cross-sections were used by a custom
MATLABTM program to measure the mediolateral location of the
trochlear sulcus, anteroposterior heights of the medial/lateral femoral
condyles, and the trochlear bisector angle (Fig. 2).
component brought the trochlear groove closer to the normal anatomy
than no external rotation; (2) however, even with external rotation the
trochlear groove in the current TKA did not fully restore normal
anatomy. This suggests that current TKA may not be designed to fully
restore or replicate normal trochlear anatomy, and other considerations
such as minimizing patellofemoral contact forces and ensuring capture
of patella in early flexion, may guide the design decisions. Therefore,
surgeons should be aware that manufacturer’s definition of anatomic
groove geometry may not imply exact replication of normal anatomy,
and exact restoration of physiologic patellar tracking may not be feasible
with current designs. Further work is needed to determine if this
compromise may be responsible for patellar complications, and how
implant designs can be modified to obtain improved performance.
Sulcus ML Location (M+, mm)
INTRODUCTION:
While total knee arthroplasty (TKA) has proven to be a highly
successful procedure, patellofemoral complications are commonly
observed. Current surgical protocol involves placement of the femoral
component in ~3° external rotation, in order to optimize patellar tracking
and to obtain balanced flexion gap. However, biomechanical studies
have shown that even with this optimal externally rotated position of the
component, patellar tracking is not restored to normal. Therefore the
present study was conducted to investigate two hypotheses: (1) External
rotation of the TKA component brings the trochlear groove closer to the
normal anatomy than no external rotation, and (2) Even with external
rotation of the component, the trochlear groove in current TKA is not
designed to fully restore normal trochlear anatomy.
-6
B
*
-8
0
20
40
60
80
C
100
θ / θmax (%)
Figure 3. Mediolateral sulcus location along the trochlear groove. (*)
Implies significant difference between normal knee (Cartilage), and
externally rotated TKA (CR TKA (3 deg)). CR TKA (0 deg) refers to
TKA with no external rotation. Patellofemoral contact locations at 0°,
30° and 105° knee flexion are indicated by vertical dotted lines.
Poster No. 2189 • 56th Annual Meeting of the Orthopaedic Research Society