Title: Measurement of posterior tibial slope (a cross-sectional study in Tehran)
Author(s): Mohamad Qoreishi MD, Syavash Hemmati MD, Ali Sina Shahi MD, Mehrnoush Hassas Yeganeh MD, Seyyed Morteza Kazemi MD*
Affiliattion(s): Shahid Beheshti University of Medical Sciences, Tehran, Iran
* Corresponding Author
Vol 2, Num 1, January 2015
Introduction:Posterior tibial slope (PTS) is the normal anteroposterior inclination of the tibia plateau and plays an important role in the biomechanics of the knee and function of the cruciate ligaments. The posterior tibial angle varies in different populations and there is no report about its measurement in the Iranian population. The aim of this study was to investigate the normal posterior tibial slope in normal people in a teaching hospital in Tehran-Iran.
Materials and Methods:In this descriptive study, 431 knees in 431 people (230 men and 201 women) with no history of prior knee problem were evaluated by a lateral knee and leg radiograph. The average age in these cases was 31.7 years (19 – 60 years). These people entered the study by their choice. The angle of tibial slope was measured in all cases and analyzed.
Results:The mean slope angle was 9.3±1.4 degrees. There was no statistical relationship between age and gender and posterior tibial slope (P<0.05).
Conclusion:If we can consider these people as a sample of Iranian population, the normal posterior tibial slope angle is different from other countries. It may be necessary to consider these differences in designing the tibial components for knee arthroplasty.
Keyword:Tibial anatomy, Slope, Knee
Stability of the knee is provided mainly by ligamentous structures and bony components play a very small role in this aspect.(1, 2) Posterior tibial slope is one of the limited bony characteristics of the knee joint that contributes to joint stability.(3, 4) The posterior inclination of the tibial plateau, or posterior tibial slope (PTS), contributes to anteroposterior (AP) stability.(5, 6)
In arthroplasty, the natural PTS should not be modified during implantation of a unicompartmental knee prosthesis (7) nor during high tibial osteotomy.(8, 9, 10) In TKA, an inappropriate cutting angle of the PTS results in polyethylene wear, component loosening, and posterior cruciate ligament (PCL) strain.(2, 11, 12)
The PTS is defined on a lateral radiograph by the angle between perpendicular to the longitudinal axis of the bone and tangent to the medial and the lateral plateaus.(13)
As far as we know, the only previous study on PTS angle in Iranian population was performed by us in a more limited number of people.(14) We have tried to complete this research with increasing the number of patients.
Materials and Methods
We assessed 431 knee joints radiographs from 431 people (230 men and 201 women) who accepted to participate in the research. We got the approval of ethics committee of the SBMU and all the people signed informed consent before participating in the research. These were all peoples with no previous history of knee problem. Exclusion criteria were all those people with history of knee surgery, knee infection, knee DJD, systemic joint diseases such as RA, limb-length discrepancy and inability to obtain true lateral radiography in the first attempt.
All the people, in whom we could not obtain true lateral x-ray on the first attempt, were excluded from the study. Our criteria for a true lateral X-ray of the knee joint was complete and perfect overlapping of the tibial plateau condyles. For obtaining this view, the X-ray beam should be centered on the tibial epiphysis.
For measuring the PTS angle, we draw two lines, one tangential to posterior tibial shaft cortex and the other perpendicular to the first line. Then we draw a third line tangential to the surface of the tibial plateau and measured the angle between this third and the second lines in degrees.
All the measurements were performed by a single orthopaedic surgeon (HRSH) in two different situations separated by 2 months. Intraobserver reliability was calculated to be 0.9.
The average age of the cases was 31.7±8.7 years (19-60 years). The posterior tibial slope angle was measured to be 9.3±1.4 degrees. The distribution of the amount of PTS is less than 5 degrees in 11% of cases, and less than 10 degrees in 59% of cases. The maximum measured PTS angle in these cases was 19 degrees. (Table 1)
The PTS in two age groups (less and more than average age) and in two sexes were compared. There is no statistically significant difference between PTS and age and sex. (P≥0.05) (Table 2)
Knowledge of anthropometric measurements of knee joint is very important in planning the treatment and designing the prostheses.(15) In fact all the orthopaedic devices and prostheses are designed based on these measurements in western Caucasian people. If there are differences between these measurements in different ethnic groups, the same devices, which must simulate the normal anatomy and biomechanics of the knee joint, will not be appropriate for every ethnic group. So performing these researches in Iranian as a subgroup of Asian people seems to be logical and necessary.
On the other hand, one of the most important factors in survival of the knee prostheses is femoral and tibial component alignment.(11,16) The component alignment is essentially effective in determining the pattern of the load distribution in polyethylene and component-bone interfaces. If this load is not correctly distributed, there will be stress concentration in an inappropriate location, leading to excessive wear or premature loosening and ultimately early failure of the prostheses. So knowing the correct inherent alignment of knee joint in each ethnic group is of paramount importance for performing a perfect TKA.(17)
Posterior tibial slope is an important parameter in knee joint biomechanics with the most effect of knee ligaments. In previous studies in other ethnic groups, Jiang et al. found PTS to be 10 degrees in 50 Taiwanese people (18) and Matsuda et al. in 30 Japanese people found it to be 10.7 degrees.(19)
Moore et al measured PTS in 50 American people and found it to be 14 degrees.(20) In the present study, we measured PTS in 431 normal Iranian people and found it to be 9.3±1.4 degrees, which is smaller than other ethnic groups, esp. western people. (Table 3) The reason why this difference happens is not clear. Probable explanations may include genetic factors, specific pattern of sitting on the ground or the particular shape of Iranian toilets.
In this study, we could find any relationship between neither age nor sex and PTS. The most important application of this finding may be during performing knee replacement, in which reconstructing the PTS is vital for obtaining complete flexion and guaranteeing the prosthesis survival.(6, 11, 12) It may be useful in designing proper knee prosthesis for Iranian population.
This study has solved a limitation of our previous study by increasing the number of cases, but there are still some limitations, the most important of which is selection of cases from one geographic region. Although our center is a referral hospital from all around the country, most of our cases were from Tehran. So we are to perform this study by including cases from all around the country.
Mohammad Qoreishi MD Orthopaedic surgeon, Assistant professor, Shahid Beheshti Medical University, Tehran, Iran email@example.com
Syavash Hemmati MD Orthopaedic surgeon, Tehran, Iran
Ali Sina Shahi MD Postdoctrate Research Fellow, Rothman Institue, PA, USA firstname.lastname@example.org
Mehrnoush Hassas Yeganeh MD Assistant professor, Paediatric rheumatologist, Shahid Beheshti University of Medical Sciences, Tehran, Iran email@example.com
Seyyed Morteza Kazemi MD Associate professor, Orthopaedic knee surgeon, Shahid Beheshti University of Medical Sciences, Tehran, Iran Corresponding Author firstname.lastname@example.org
Acknowledgements: None declared.
Financial disclosure: None declared.
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