AAOS Now

Published 6/1/2016
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Maureen Leahy

The Relationship Between Tibia-Femur Length Ratio and OA

Study examines implications for the spine, hip, and knee

Results of a study presented at the 2016 AAOS Annual Meeting in Orlando, Fla., revealed that a tibia-femur (T/F) length ratio greater than 0.80 was a significant predictor of osteoarthritis (OA) in the hip and knee.

According to Raymond W. Liu, MD, who presented the research findings, this can be important information for orthopaedic surgeons who perform limb-lengthening procedures.

Although surgical limb-lengthening or limb-shortening correction in the lower extremities can occur at the femur, tibia, or in both bones, "little data exist about the long-term consequences of T/F length segment disproportion in limb-equalization surgery," said Dr. Liu.

"The purpose of our study was to confirm the normal T/F ratio—believed to be 0.80—and determine if T/F ratio correlates to degenerative joint disease of the spine, hip, and knee," he said.

Cadaveric study
The study involved 576 adult skeletons (86 percent male; average age: 56 years; average height: 1,708 mm) acquired from the Hamann-Todd Osteological Collection at the Cleveland Museum of Natural History. Specimens with traumatic, metabolic, or infectious diseases were excluded.

Using a digital ruler, the researchers measured the bone lengths of the 1,152 tibiae and femora. Tibia length was measured from the lateral tibial plateau to the lateral tibia plafond; femoral length was measured from the top of the femur head to the femoral condyles. The T/F ratio for each specimen was then calculated by dividing tibia length by femur length.

The researchers also graded each specimen's spine, hips, and knees for OA, based on the presence or absence of OA's known osteological signs. Using primarily osteophytic patterns, the investigators graded spine OA from 0 (normal) to 4 (very arthritic). OA grading measurements of the proximal femur and acetabulum were combined to form a composite hip measurement, graded from 0 to 6. OA grading measurements of the patellofemoral, medial, and lateral knee compartments were averaged to form a composite knee measurement, graded from 0 to 6. Each researcher also independently measured 20 specimens to establish inter- and intra-relator reliability.

Results
The average T/F ratio among the specimens was 0.80. The inter- and intra-relator reliability of OA grading was 0.60 or higher, demonstrating good agreement. "In addition, OA in the spine, hips, and knees increased linearly with age, which also validated our grading," said Dr. Liu.

Multiple regression analysis also revealed a slight correlation between male sex and increased spine OA, and increased height and knee OA. Although there was no strong correlation between T/F ratio and spine OA, the investigators found that an increased T/F ratio was a significant predictor of OA in the hip and knee (P = 0.006 and P = 0.008, respectively).

"This was the most important finding of the study—that a relatively long tibia was associated with hip and knee OA," said Dr. Liu.

Although the exact mechanism for the increased development of hip and knee OA in the specimens is unknown, the researchers suspect it may be related to altered gait.

The researchers further analyzed their findings by dividing the specimens into the following two groups: those with a T/F ratio above 0.80 and those with a T/F ratio below 0.80.
"We found that although a T/F ratio above 0.80 is associated with OA in the hip and knee, a T/F ratio below 0.80 does not add any protective benefit," said Dr. Liu. "When equalizing the lower extremity, therefore, we think it is reasonable to aim for a T/F ratio of 0.80 or to match the ratio of the opposite limb.

"When that is not possible," he added, "preference should be toward lengthening the femur or shortening the tibia to help prevent the potential development of OA."

Dr. Liu's coauthor of "The Association of Tibia Femur Ratio and Degenerative Disease of the Spine, Hips, and Knees," is Douglas S. Weinberg, MD.

The authors' disclosure information can be accessed at www.aaos.org/disclosure

Maureen Leahy is the assistant managing editor of AAOS Now. She can be reached at leahy@aaos.org

Bottom Line

  • Data on the long-term consequences of T/F length segment disproportion in limb-equalization surgery are limited.
  • This study sought to determine whether an association exists between T/F ratio and degenerative joint disease of the spine, hip, and knee.
  • The researchers identified a strong correlation between a T/F ratio above 0.80 and hip and knee OA.
  • This information may aid limb-lengthening surgeons in surgical planning.