Lower limb alignment, joint orientation and tibial malunions

              Lower limb alignment, joint orientation and tibial malunions 

                                                           Dr KS Dhillon

What is normal lower limb alignment and joint orientation?

The lower limb alignment is determined by the orientation and shape of the the femur and the tibia as well as the orientation of the hip, knee and ankle joints. The femur and the tibia each has two axis, the mechanical and the anatomical axis.
The mechanical axis runs from the centre of the proximal joint to the centre of the distal joint and this axis remains the same in the sagittal and frontal planes. The anatomical axis on the hand runs through the middle of the diaphysis of the long bone and it can be straight in both planes as in the tibia or it can be straight in one plane (frontal) and curved in another plane (sagittal) as in the femur. For practical purposes the mechanical axis of the lower limb is always considered in the frontal plane.
The angle which is formed between the joint line and the anatomic or mechanical axis is known as the joint orientation angle. The normal lateral distal femur angle (mLDFA) between the femoral mechanical axis and the knee joint line is 87 degrees and between the knee joint line and the femoral anatomical axis (aLDFA) is 81 degrees (79 to 83 degrees).
The medial proximal tibial angle (MPTA) is 87 degrees (85 to 90 degrees) both with the anatomical and mechanical axis because both these axis are the same.
The normal angle between the femoral and tibial mechanical axis was always believed to be 0 degrees. Eckhoff DG et al [1], however, have challenged the ‘concept of a mechanical axis consisting of a straight line through the centers of the femoral head, distal aspect of the femur, and the
talus’.
In a study of the three-dimensional mechanics, kinematics, and morphology of the knee they found that only 2% of individuals have a neutral hip-knee-ankle axis. This is probably due to the the wide variability of the bow in the tibia and femur and the lack of correlation between the bow of the tibia and femur in a given limb. They found that of the 180 subjects studied 57% had a varus angulation at the knee and 40.5% had a valgus angulation. The measurements ranged between 12.2 degrees varus to 15.6 degrees valgus. The median varus however was 2.5 degrees and the median valgus was 1.5 degrees.
Short Xray films of the knee are often used to evaluate the lower limb alignment. Howell et al [2] studied the longitudinal shapes of the tibia and femur and found that they are unrelated and are variable. They measured the angle formed by the anatomic axis of the proximal fourth of the tibia and the mechanical axis of the tibia, as well as the angle formed by the anatomic axis of the distal fourth of the femur and the mechanical axis of the femur and they were able to study the bow of the femur and the tibia in normal individuals.
They found that the angle formed by the anatomic axis and the mechanical axis of both the tibia and femur varied widely and the bow of the tibia and femur varied widely. The angle formed by the tibial mechanical and anatomic axis varied 11 degrees from -4 degrees to 6 degrees. The angle formed by the anatomic and mechanical axes of the femur varied 10 degrees from -1 degree to 8 degrees [2].
The bow of the tibia (the offset of the anatomic axis from the center of the talus) varied 5.7 cm and the bow of the femur (the offset of the anatomic axis from the center of the femoral head) varied 7.2 cm. 
Tang et al [3] found that in the chinese population the angle between the femoral and tibial mechanical axis was not 0 degrees. There was a larger medial inclination of knee joint (varus) in their subjects. Their females had a significantly larger varus alignment of the lower extremity as compared to the caucasian population. They also found that the medial inclination of the tibial plateau was 5.4 ± 2.5 degrees for women and 4.9 ± 2.3 degrees for men which was greater than the commonly reported 3 degrees.
Felson et al [4] did a study to find out if anatomic alignment measured from a knee radiograph can substitute for mechanical alignment from full limb films. They studied 143 subjects who had knee X-rays and full limb films. They found that that the agreement of alignment from knee X-ray to full limb film was only moderate. The anatomic alignment as assessed from the knee radiograph was not exactly the same as the mechanical alignment as measurement from the full limb x-rays. There were knees that were valgus on the knee X-ray that were varus on full limb film and vice versa.

Tibial malunions

The incidence of malunion after treatment of tibial fractures varies between 0% to 31.7% [5]. Court-Brown et al [6] reported a malunion rate of 2.4% in a study of 125 closed and type I open tibial fractures which were treated by nailing. Malunion was defined as shortening of more than 1 cm and or more than 5 degrees of angulation or rotation.
Freedman and Johnson [7] found 12% malalignment in 133 fractures of the tibial treated by nailing. They defined malalignment as 5 degree angulatory deformity in any plane. Malalignment was seen in 58% of proximal third fractures, 7% of middle third fractures, and 8% of distal third fractures.
Bedi et al [8] report a 13.8% incidence of malunion in patients who had treatment for tibial fractures.
Vallier et al [9] reported a 29% incidence of more than 5° of malalignment after nailing as compared with 5.4% after plating of tibial fractures.
Abdel-Salam et al [10] reported a 15.5% incidence of malunions in patients who had plaster cast treatment for tibia fractures. They however defined malunions as angulations of more than 10° or a 2.5 cm or more of shortening.
Jindal [11] reported a malunion incidence of 32% in patients with fracture of the tibia who were treated with a cast.
Wiss et al [12] followed up 101 patients with fracture of the tibia treated by nailing. They found a 4.9% incidence of malunion of the tibia. They defined malunion as an angulation of more than 7° or a more than 1 cm of shortening.
Gregory et al [13] followed up 38 patients with fracture of the tibia treated by nailing and they found a malunion incidence of 7.9%. They defined malunion as angulation of 5° or more and rotation of 10° or more and 1 cm or more of shortening.
A review of literature shows that the criteria for malunion varies, ranging from 5° of varus, valgus, and anterior or posterior angulation, 1 cm of shortening, and 5° of rotation in most studies, to as high as 10° of angulation and 2.5 cm of shortening [5].

Long term outcome of tibia malunion

Good intermediate-term results of treatment of tibial fractures have been reported in majority of the patients in an era when most tibial fractures were treated conservative with a plaster cast [14]. Tibial malunions are more come when fractures are treated with a cast as compared to when they are treated with internal fixation. Tibial malunion has been shown to increase contact stresses in the medial or the lateral compartment of the knee depending on whether it varus or valgus malunion [15]. Similarly contact stress can increase in the ankle when there is malunion of the tibia and there have been suggestions that these increase in stresses can lead to osteoarthritis (OA) of the joints [15,16].
There have been several clinical studies published which showed no association between malunions of the tibia and OA of the knee and ankle [17,18].
Van der Schoot et al [19] on the other hand showed that in patients with more than 5 degree angular malunion of the tibia there was a higher incidence of knee and ankle OA as compared to the uninjured side. They found a significant correlation between symptoms in the knee and arthritis but there was no significant relationship between symptoms and ankle arthritis or malalignment.
A more detailed examination of the paper shows that the clinical outcome at an average follow up of 15 years was good in the patients. They reviewed 88 patients out of 106 who were treated for a fracture of the tibia. They found that 49% of the patients had 5 degrees or more of angular deformity and out of these patients 58% had OA of the knee and or ankle. Thirty-one percent of the patient with no malunion had OA. The incidence of grade 2 to 3 OA was low. For the knee it was 4.5% and for the ankle it was 14.5%. Only 13.5 of the patients had symptoms of the knee or ankle or both. About 6.5% of the patients complained of pain at the fracture site. The relationship for symptoms and OA was significant only for the knee and not for the ankle. There was no mention of further surgery for the OA in this report.
The authors concluded that despite the high incidence of OA, there was no relation between malalignment and clinical symptoms. The OA could also be partly due to trauma to the joints at the time the fracture occurred.
The most comprehensive, though retrospective, report, on the long term outcome of tibial fractures which addresses the the issue of malunions is the one by Milner et al [20].
They assessed 164 patients who had tibial shaft fractures 30 to 43 years prior to the review. The subjects were evaluated with regards to lower limb joint pain, stiffness, and disability (assessed with WOMAC osteoarthritis questionnaire); clinical signs of osteoarthritis; and radiographic evidence of knee, ankle, and subtalar joint OA.
Fifteen percent of the patients reported at least moderate knee pain, 6% at least moderate ankle pain and 13% reported at least moderate disability. They found that knee OA was frequently bilateral. The study showed no significant univariate association between malunions of the tibia and the development of osteoarthritis.
The malalignment was assessed using the mechanical hip-knee-ankle angle outside the normal range of 6.25° of varus to 4.75° of valgus. Overall malalignment was seen in 15% of the subjects (17 patients). In about half of the patients (9 patients) the malalignment was due to the fracture malunion and in the rest (8 patients) the malalignment predated the fracture. Most of the subjects in whom OA was observed had normal overall alignment of the lower limb.
The authors concluded that the outcome of treatment tibial shaft fractures is good at thirty-year follow up despite the fact that mild OA is common. They also concluded that fracture malunion is not the cause of the higher prevalence of symptomatic ankle and subtalar osteoarthritis in the injured limb. Varus malalignment occurs occasionally and may produce OA of the medial compartment of the knee, but other undefined factors, rather than malalignment due to malunion, are responsible for the OA after tibial shaft fractures.
Lefaivre et al (21) reported a 14 years follow up of a  small series of patients with fracture of the tibia who were treated by nailing. They found a 35.4% incidence of OA despite the absence of radiographic malalignment. The incidence of knee OA was 16.1%, ankle OA also 16.1%  and 3.2% of the patients had both knee and ankle OA despite the absence of malunion.

Treatment of tibia malunion

The precise definition of a malunion remains elusive and the limits of deformity which can lead to OA also remains imprecise. A tibial malunion, however, can result in angular, rotational and or transitional deformity as well as shortening or lengthening of the bone. When the deformity and or limb length inequality should be corrected remains unresolved. Generally it is believed that surgical intervention is necessary when there is functional (limp, pain) and or cosmetic disability (mark deformity) [22]. Pain at the knee and ankle due to the malunion is relatively uncommon [23].
Though tibial malunion is quite common, treatment of malunion is not often described and the patient numbers are small in the published reports [23].
There remains little agreement in literature as to the degree of limb length inequality that is clinically significant. Most authors agree that the current indication for lengthening of the lower limb would be a disparity exceeding 5 to 6 cm [24]. Usually limb length inequality of 2 cm or less does not require any treatment because such discrepancies are well tolerate [25]. Limb length inequality of between 2 cm and 5 cm is usually treated with a shoe raise.
Correction of length can be carried out by compression osteotomy for shortening of bone and a distraction osteotomy for lengthening the bone. Large length discrepancies can be corrected by gradual distraction, following a metaphyseal corticotomy, using external fixators. The gradual distraction can also correct angular, rotational and translational deformities.
Angular and rotational deformities are corrected with an osteotomy at the level of the deformity followed by internal or external fixation. Several techniques for correction of tibial malunion have been described which can  achieve excellent results but surgery can be associated substantial risk and recovery time. These risks should be discussed at length with patients when planning such surgery [22].

References

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2. Howell SM, Kuznik K, Hull ML, Siston RA. Longitudinal shapes of the tibia and femur are unrelated and variable. Clin Orthop Relat Res. 2010 Apr;468(4):1142-8.
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