a Departamento de Ortopedia, Santa Casa de São Paulo, São Paulo, SP, Brasil
b Serviço de Ortopedia e Traumatologia, Hospital Israelita Albert Einstein, Universidade Federal de São Paulo (Unifesp), São Paulo, SP,Brasil
c Instituto de Ortopedia, Hospital das Clínicas, Faculdade de Medicina, Universidade de São Paulo (USP), São Paulo, SP, Brasil
d Escola Paulista de Medicina, Universidade Federal de São Paulo (Unifesp), São Paulo, SP, Brasil
Introduction
Numerous studies have shown a correlation between thedurability of total knee arthroplasty (TKA) and restoration ofthe normal limb alignment.1-3It is believed that restoration ofthe mechanical axis with a maximum variation of 3?towardvarus or valgus is associated with the best results from TKA.1-6However, some authors have demonstrated that postoperativealignments of the limb outside of the interval of ±3?in thecoronal plane are observed in up to 30% of the cases.
9In normal knees, the tibial joint surface is at a varus angle ofapproximately 3?in relation to the mechanical axis, while thefemoral surface is at a valgus angle of 9?. Historically, attemptshave been made to reproduce this anatomical alignment ofthe knee in total arthroplasty by cutting the tibia at a varusangle. However, several studies have demonstrated that tibialcomponents placed at varus angles greater than 5?tend to faildue to medial collapse.
Incorrect alignment of TKA has been identified as a causeof long-term complications, including accelerated wear,11,12premature mechanical loosening of the implant1,13,14andpatellofemoral problems15-17such as patellofemoral instabil-ity and patellar fracture.
Thus, it is recommended that the tibial component shouldbe implanted perpendicularly to the mechanical axis of thetibia in the coronal plane. The femoral component is usuallyimplanted at a valgus angle of 5?to 6?, which is the size of anglesupposedly necessary for reestablishing a neutral mechanicalaxis in the limb.
The aim of the present study was to measure the ideal anglefor making the distal femoral cut in Brazilian patients who
underwent TKA at the Knee Surgery Center of the NationalInstitute of Traumatology and Orthopedics (INTO).
Material and methods
Between August 2011 and February 2012, panoramic radio-graphs on 79 patients (22 men and 57 women) were analyzed,thus totaling 107 limbs, in accordance with the inclusion andexclusion criteria listed in Table 1.
This study was submitted for evaluation and approval byour institution's research ethics committee.
Radiographic evaluation
The radiographic evaluation was done on anteroposterior (AP)panoramic radiographs of the lower limbs with weight-bearingon both feet. All the radiographs were produced at the imag-ing examination center of our service. The examinations were
performed with the patients positioned with their limbs atneutral rotation and maximum extension.
In all the radiographic examinations, we defined: (1) theanatomical axis of the femoral diaphysis; (2) the mechanicalfemoral axis; and (3) the cervicodiaphyseal angle.
The mechanical axis of the femoral diaphysis was definedby a straight line that joined the centers of two circles thatwere tangential to the medial and lateral cortical bone of thefemur. The first circle was located 2 cm distally to the lessertrochanter. The second was at the junction between the distalmetaphysis and the femoral diaphysis, as determined usingHeim's square.
The mechanical axis was defined in accordance with thecurrent concepts in the literature, as a straight line passingthrough the center of the femoral head to the midpoint of thewidth of the distal femur.
The ideal distal femoral cut angle corresponded to theintersection between the anatomical axis and the femoralmechanical axis (Fig. 1).
Measurement of the angle formed between the femoralanatomical axis and the line of the femoral neck defined thecervicodiaphyseal angle. Two circles that were tangential tothe lower cortex and upper cortex of the femoral neck wereused to trace out the line of the neck.
The measurements were always made by two evaluators atdifferent times using the same instruments with precision ofthe order of millimeters.
Statistical analysis
Statistical analysis was performed with the aim of evaluatingthe degree of significance of the parameters measured. TheShapiro-Wilks W test was used to evaluate whether the vari-ance had normal distribution and the Levene test was usedto assess its homoscedasticity. The distal femoral cut anglesand the cervicodiaphyseal angles were considered to have nor-mal distribution and Student's t test was used to compare themeans. The Statistica 8.0 software was used for the statisticalcalculations.
Results
Seventy-nine patients (22 men and 57 women) were studied,with a total of 107 limbs. The patients' mean age was 67 years,with a range from 58 to 86. Surgery was performed on the rightside in 53 cases and on the left side in 54 cases.
Seventy knees presented alignment with varus anglesbetween 3?and 20?(mean 8.4?; standard deviation 3.5?).Twenty-six knees presented preoperative alignment with val-gus angles between 2.7?and 16?(mean 6.6?; standard deviation3.1). Neutral preoperative alignment was observed in 11 cases.
The ideal femoral valgus angle ranged from 4.2?to 8.6?,with a mean of 6.3?. Fig. 2 shows the distribution of the idealdistal femoral cut for the patients studied.
The male patients presented an ideal distal femoral cut of6.6?(range from 4.9?to 8?), while for the women, 6.2?was theideal angle for the distal femoral cut (range from 4.2?to 8.6?).
Among the patients with preoperative varus alignment,the ideal distal femoral cut was 6.2?(range from 4.2?to 8.4?).Among the patients with preoperative valgus alignment, theideal femoral valgus angle was 6.5?(range from 4.2?to 8.6?).
The cervicodiaphyseal angle ranged from 114.3?to 138.3?,with a mean of 127.2?. Fig. 3 correlates the distal femoral cutand cervicodiaphyseal angle values for each patient.
Among the male patients, the mean cervicodiaphysealangle was 127.5?(range from 118.1?to 138.3?), while amongthe women it was 127?(range from 114.3?to 136.5?). The meancervicodiaphyseal angle identified among the patients withpreoperative varus alignment was 127.2?(range from 114.3?to 136.5?), while among those with preoperative valgus align-ment, it was 127.1?(range from 115.1?to 138.3?).
Statistical analyses were performed using Student's t testwith a 95% confidence interval. No statistical difference in theideal distal femoral valgus values or the cervicodiaphysealangle was observed between the men and women (p = 0.57).The statistical analysis on these angles also did not show anystatistical difference in comparing the preoperative varus andvalgus deformities (p = 0.18).
Discussion
The analysis on the ideal femoral valgus angle showed smallabsolute angle values. Thus, precise radiographic standardsneed to be used in panoramic radiographs, especially withregard to controlling the external rotation of the lower limbswhile the examination is being performed. Radiographs withrotational deviation of the lower limbs, which most frequentlyoccurs during external rotation, produce larger femoral val-gus angles because of the anatomical bowing of the femuralong the sagittal axis. This bowing also impedes proper mea-surement of the anatomical axis of the femoral canal. For thisreason, only radiographs with perfect rotational control, suchthat the lesser trochanter did not appear and the patella wascentralized on the knee, were included in this study.
Another complicating factor in determining the angles wasextra-articular deformity, which altered the axes and anglesanalyzed unpredictably. Radiographs with extra-articulardeformity were excluded from the study.
One of the objectives of TKA is to restore neutral alignmentof the lower limb through making bone cuts perpendicularlyto the mechanical axes of the femur and tibia.20It is a commonpractice among many surgeons to use the same distal femoralcut angle for all patients and to assume that there is minimalvariation in the angle between the mechanical and anatom-ical axes of different patients' knees. However, some studieshave advocated preoperative planning using panoramic radio-graphs as a means of obtaining alignment that is more preciseand individualized.
21,22The mean value of the distal femoral cut that was foundfor the present study population did not show any statisticaldifference in relation to what was found by Resende et al.23in another Brazilian population. If we had empirically usedthe mean angle found for the patients in this study, all ofthem would have had acceptable alignment, while taking intoaccount a permissible error of up to 3?, as put forward in theliterature.24,25This diverges from the data of one Brazilianauthor, who found that 19.7% of the population operated hadinsufficient alignment, based on an acceptable error of 3?inthe coronal plan.
Despite a tendency for the distal femoral cut to be greater inmen than in women (6.6?versus 6.2?), there was no statisticaldifference between the groups, which is concordant with thecurrent literature
We identified an inverse trend between the cervicodiaphy-seal angle values and the ideal distal femoral cut values. Thiswas due to the greater distance of the diaphysis from the cen-tral axis of the body in the femoral necks with greater varusangle and the smaller distance of the diaphysis from the cen-tral axis of the body in the femoral necks with greater valgusangle.
The preoperative coronal alignment did not significantlycorrelate with the distal femoral cut in this study. The distalfemoral cut value was related to the anatomical factors of thefemur, without using any tibial parameter to determine it. Thismakes us think that the overall alignment of the limb doesnot influence the distal cut. On the other hand, Deakin et al.20demonstrated a relationship between the distal femoral cutand the alignment of the lower limb, which should be lessthan 6?in valgus cases and greater than 6?in severe varuscases.
Conclusion
The mean angle between the femoral mechanical axis and thefemoral anatomical axis was 6.3?.
The preoperative coronal alignment and sex did not haveany influence on the distal femoral cut.
The cervicodiaphyseal angle had an inverse relationshipwith the distal femoral cut.
Conflicts of interest
The authors declare no conflicts of interest.
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