Analysis on anatomical references to assess the coronal alignment of tibial and femoral cuts in mega prosthetic knee replacement
© The Author(s) 2013
Received: 30 March 2013
Accepted: 11 November 2013
Published: 21 December 2013
In megaprosthetic knee replacement, surgeons use cutting guides that depend on anatomLevel of evidence
ical references to determine the ideal cutting plane alignment. In this work, we investigated the accuracy of using femoral cortical surfaces and tibial canal portions as the references. The study aims to improve the design and use of the cutting guides.
Materials and methods
Sixty-one knee scanograms of 33 patients (mean age around 20 years) diagnosed with osteogenic sarcoma and undergoing distal femur megaprosthetic surgery were acquired. Angles between the selected anatomical references and axis perpendicular to the ideal cutting plane (anatomical axis for femur and mechanical axis for tibia) were measured for both femur and tibia, in coronal view. The smaller the magnitude of the angles, the better the anatomical reference is.
At the central femoral region, on average, both lateral and medial cortical surfaces give accurate alignment of the ideal cutting plane (0.6° and 0.8°, respectively), with no significant difference (p > 0.01). At the distal region, the lateral cortical surface gives significantly better alignment compared to the medial cortical surface (p < 0.01), but not as accurate (1.4°) as in the central region. For tibia, the central tibial canal gives significantly accurate alignment of the ideal cutting plane (−0.3°) on average, compared to the proximal tibial canal (p < 0.01).
For a femoral cut, both lateral and medial cortical surfaces are the best anatomical references, but only at the central region. For a tibial cut, the central anatomical axis is the best reference.
Level of evidence
For both femoral and tibial cuts, it is unclear which anatomical reference, when being referred by the cutting guides, will result in more accurate bone cuts. Therefore, we investigated and compared the accuracy of using femoral cortical surfaces (medial and lateral) and tibial canal portions (proximal and central) as references to assess the alignment of the ideal cuts in the coronal plane, for megaprosthetic knee replacement.
Materials and methods
The study was performed on randomly accessed medical preoperative scanograms (in the coronal view) of Indian patients diagnosed with osteogenic sarcoma, undergoing distal femur megaprosthetic knee replacement surgery. In the femoral study, 61 knees in scanograms of 33 patients were used. This study group consisted of 19 males (mean age 18.6 years; range 11–50 years) and 14 females (mean age 20.6 years; range 13–40 years). In the tibial study, 59 knees in scanograms of 30 patients were used. This study group consisted of 17 males (mean age 21.4 years; range 11–50 years) and 13 females (mean age 17.2 years; range 13–40 years).
Baseline patient characteristics
Number of knees
Number of patients
Mean age, SD (years)
Median age (years)
Range of age (years)
Anatomical references selected and measurements performed in the study
Angles measured between
Medial femoral cortical line (medial FCL) and femoral anatomical axis (FAA)
Lateral femoral cortical line (lateral FCL) and femoral anatomical axis (FAA)
Proximal anatomical axis (PAA) and mechanical axis (MA)
Central anatomical axis (CAA) and mechanical axis (MA)
For all the parameters, values of the mean, standard deviation, median, and interquartile range were calculated. Three null hypotheses were stated: (a) there is no difference in the accuracy of using lateral and medial cortical surfaces as anatomical references to determine the ideal femoral cut at distal region of femur; (b) there is no difference in the accuracy of using lateral and medial cortical surfaces as anatomical reference to determine the ideal femoral cut at central region of femur; and (c) there is no difference in the accuracy of using proximal and central portion of tibial canal as anatomical reference to determine the ideal tibial cut. The angles between the axis perpendicular to the ideal cut (FAA in femur and MA in tibia) and various anatomical references (medial/lateral FCLs for femur and CAA/PAA for tibia) were compared by the Wilcoxon matched-pairs signed-rank test. The two-sided level of significance was kept as 0.01. The mathematical calculations and the statistical analysis were performed using Microsoft Excel 2007.
Measurements of angles between selected anatomical references and axis perpendicular to the ideal cutting plane
Lateral FCL–FAA mean (SD)
Medial FCL–FAA mean (SD)
Significance p value
Femoral anatomical references
Distal portion (14 cm from distal end)
Central portion (21 cm from distal end)
0.6 (−0.4 to 1.4
0.8 (0 to 1.5)
CAA–MA mean (SD)
PAA–MA mean (SD)
Significance p value
Tibial anatomical references
0.8 (−0.7 to 0.3)
−0.2 (−0.4 to 1.7)
Lateral and medial FCLs were chosen as anatomical references in this study, because using the bone surface directly as a reference and cutting the bone perpendicular to it would be the easiest approach for cutting guide design. The results show that at the central region of the femur, both the cortical surfaces (medial and lateral) can be accurate references (no significant difference, p > 0.01). At the central region, the mean angle between FCL and FAA was < 1° with standard deviation of around 1.5°. At the distal region, though the alignment of the lateral FCL is closer to that of FAA compared to the medial FCL (p < 0.01), the mean angle between them is more than 1° with a large standard deviation of 2°. Hence, both lateral and medial cortical surfaces cannot be used as the reference for accurate cut at distal femoral region.
For tibia, Yoo et al.  had shown that in the sagittal plane, the PAA is nearly parallel to the MA, and suggested using the PAA as a sagittal plane reference with intramedullary cutting guide. In our work, anatomical references were studied for the coronal plane alignment. Both proximal and central portion of the tibial canal were selected as the references. These references can be accessed by the intramedullary rod, depending on its length and the position of an entry hole. In our study, the central anatomical axis was significantly closer to that of the MA of the tibia compared to PAA (p < 0.01). Hence, CAA appears to be the best anatomical reference for tibial cut.
Measurements of angles between the suggested anatomical references and axis perpendicular to the ideal cutting plane with their advantages
Suggested anatomical reference
Angle bisector of lateral and medial cortical lines (FCAB)
Central part of tibial canal accessed through an entry point at the knee center
Mean angle between anatomical reference and axis perpendicular to ideal cutting plane
Distal region: 0.7° (SD = 1.4) Central region: 0.1° (SD = 1.2)
0.5° (SD = 0.6)
Most accurate at both distal and central region
No need of preoperative plan to determine the entry hole
x = 10 cm (in this study)
b = length of intramedullary (IM) rod
α = angle PAA–MA
β = angle between IM rod and mechanical axis (MA)
γ = angle CAA–MA
The measurements were performed in the scanograms of patients of Indian origin, and hence the results may not be applicable to a different ethnicity. Another limitation is that the study was performed using two-dimensional (2D) X-rays where the rotational positioning of the implant component cannot be studied. Berhouet et al. had presented a study based on three-dimensional (3D) computed tomography (CT) scans, where using the rotational alignment of femoral component as a reference for the rotational alignment of the tibial component was analyzed . The 3D imaging and representation enables viewing the anatomy more accurately and realistically. Intraoperative navigation based on 3D bone models are hence generally assumed to be superior to the conventional surgical guides [1, 17]. Recent studies, however, show that there is no difference in clinical function, alignment and survivorship of the components between the knees that underwent computer-navigated surgeries and those that underwent conventional surgeries [9, 16]. The conventional instruments are accurate from an engineering point of view, and should be improved in terms of how are they employed . Our study is an attempt to improve the performance of the conventional instruments by using the selected best anatomical references.
In conclusion, for a femoral cut, both lateral and medial cortical surfaces can be used as the anatomical references, but only at the central region. For both central and distal femoral cuts, the bisector of angle between the cortical lines is nearly parallel to the anatomical axis, which suggests designing a new cutting guide that can determine the angle bisector of medial and lateral cortical lines by using them as indirect references. For the tibia, the central anatomical axis can be used as a reference to get an accurate tibial cut. The central part of the tibial canal accessed through an entry hole at the knee center is also suggested as a reference in the absence of preoperative plan.
The authors would like to thank Dr. Manish Agarwal (Consultant oncologist, Hinduja hospital, Mumbai, India) for providing us the medical image data of the patients’ undergone megaprosthetic knee replacement surgeries, performed by him at Tata Memorial Hospital, Mumbai, India.
Conflict of interest
The authors declare that they have no conflict of interest.
This article is published under license to BioMed Central Ltd. Open AccessThis article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.
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