Keywords

With acquisition of the erect posture, limitation of the extension of the hip appears in human development . There are three steps in this process (Fig. 1):

Fig. 1
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(1) Active retroversion of the pelvis will help straighten the spine, but it is limited by the extension of the hip. This limit is represented by the tension of the capsule of the hip joint, the quadriceps and psoas muscles anteriorly. (2) The second step is to introduce a lordosis that will permit balance in standing position. However, this will not be enough. (3) A third step is needed with more lordosis that will create the reserve of hip extension

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The reserve of hip extension can be defined as the amplitude of maximum extension of the coxo-femoral articulation relative to the vertical neutral position.

This reserve of hip extension is necessary, because if there is a limitation during displacement, the femur would force the pelvis, by ligamentary and muscular tension, to have bending movements , with every step. Thus, it is through repeated hyperlordotic movements that the subject can maintain the fluidity of movements of walking, but when used regularly, it will lead to rapid degeneration of the spine and correlate with low back pain as has been demonstrated by several authors [1,2,3,4] (Fig. 2).

Fig. 2
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With repeated end-range extensions of the hip, it forces anteversion of the pelvis and hyperlordotic movements of the lumbar spine, with ensuing degeneration and low back pain

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This dynamic element must be taken into account in the assessment of clinical and morphological status of patients and in the assessment of abnormalities of sagittal balance.

If there is a sagittal imbalance , it can be represented by two components: anterior spinal imbalance and femoral flexion. To calculate the necessary correction, one must add these two components. However, the subject has a reserve extension which can be called “Actual Extension Reserve ”, which should be subtracted from the formula. This formula, in this representation, will allow only a balanced upright position and, as we have seen, a reserve extension is still necessary. This reserve extension, can be called “Theoretical Extension Reserve ”, should be added to the formula [5] (Fig. 3).

Fig. 3
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Calculation of necessary correction of Sagittal Imbalance

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What is the Theoretical Extension Reserve which is necessary for smooth pelvic movement? We do not have a precise figure but based on optoelectronic studies [6,7,8], we can predict that slow walk requires 10° of extension reserve, for fast walking 15° and to run at least 20°.

This formula which makes it possible to calculate the correction angle necessary in the case of a sagittal imbalance, taking into account the extension reserve, does not give the information if the correction must be made at the level of the spine or pelvis, or even at the femurs. The contribution that can be obtained by stretching was not taken into account either. Thus, in practice, if a correction is necessary, one must assess whether a correction pre- or post-operative stretching could be performed, and this figure is expected to be subtracted from the formula.

How to measure the Extension Reserve ? There are two traditional ways: clinical examination and optoelectronics. Clinical examination does not make the difference in a precise way between extension movement at the level of the hip joint or in the lower lumbar spine. Optoelectronics examination is more accurate in lean subjects; however, its application is not feasible in daily clinical practice.

We propose an original method of radiological measurements using specific radiograph images [9]. In Study 1, we compared 37 patients using two methods of measuring the extension reserve. The first method is to achieve an active retroversion movement of the pelvis in an upright position relative to the femur, while the second is to adopt a lunge position where, conversely, it is an active extension of the femur relative to the pelvis that will be applied.

A lateral radiograph with overlapped femoral heads and with a sufficient portion of the femurs as well as the lumbosacral junction allows to calculate the pelvi-femoral angle [10] in the upright neutral position and in two positions tested.

This study shows that the active pelvic retroversion method gives a significantly inferior result compared to the lunge position method. In addition, the implementation of active movement by retroversion of the pelvis presents difficulty for many patients with paradoxical results and measurement failures (Fig. 4).

Fig. 4
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Profile positions for the neutral pelvis, retroversion and lunge positions. The lunge reliably demonstrates information on reserve of extension

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We concluded that only the lunge position radiograph should be used in practice. This is understandable because this is the recommended position for stretching to improve the performance of runners with improving of passive extension of the hip joint.

This is a dynamic radiological assessment that can be criticized for its reliability, repeatability, and the influence of pain. However, this assessment can give information on the reserve of extension so far unknown in clinical applications. Therefore, we have decided to make this assessment systematic for each patient where lumbar surgery could change the sagittal balance. The exploration includes full spine and spinal dynamic radiographs, X-ray in the lunge position for right and left femur as well as in neutral position (Fig. 5).

Fig. 5
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Series of radiographs, clockwise from top left, postero-anterior views with mild coronal plane abnormality, neutral and retroversion views of the pelvis, full spin radiographs in frontal and sagittal planes, lunge views left and right and lateral views in extension and flexion

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We performed a first analysis on a series of 150 patients. Table 1 shows the results of measurements of sagittal balance.

Table 1 150 patients with measurements of sagittal balance 
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The measurement of the extension reserve for the right hip in this series is 11.6° and left hip 12.9°. The calculation of the required correction of the defect of sagittal balance is 3.4°. We found a correlation between the angle of correction needed and the age, the angle of lordosis and the imbalance (the angle compared to “plumb-line”). On the other hand, we have not found any correlation between the extension reserve and spinal or pelvic parameters.

Correlations between the correction needed calculated and the different parameters (Table 2).

Table 2 Correlations between correction and age/lordosis/plumb line
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The extension reserve appears in this study as a single factor that is not closely related to the parameters of sagittal balance.

The study of our patients revealed some specific pathologies. Several patients were found with unequal leg length from a developmental dysplasia in the hip and a limitation of the reserve extension of the dysplastic side, which leads to low back pain. Early orthotic correction of leg length and a stretching programme is hoped to avoid developing back problems in these cases. For some patients, knee conditions with stiffness can also lead to back pain.

Applications

Stretching

In the literature, it has been reported that stretching programmes provide an improvement in low back pain [11, 12]. However, the angular improvement remains very limited, as has been reported in a series by Kerrigan et al. [13] where a self-rehabilitation protocol over 10 weeks showed only 1.6° improvement on average.

Thus, the rehabilitation with relaxation of sub-pelvic segments appears useful and this programme could be considered in each case where the reserve seems limited, but it appears that one cannot count only on rehabilitation to compensate for extension deficits beyond a certain limit.

Surgery

  1. 1.

    Prevention is mandatory. To limit the loss of the extension reserve, one must avoid surgical fusions with decreased lumbar lordosis. When positioning the patient for lumbar fusion, femoral flexion should be avoided because in this position a loss of lordosis will occur which not only decreases the patient’s extension reserve and disrupts sagittal balance, but in addition limits the possibilities of compensation for the lumbar spine. This may be considered as one of the causes of adjacent segment syndrome and may partly explain why these syndromes appear more frequently after posterior than anterior surgery. In the case where long instrumentation is performed, and specifically with lumbo-pelvic fixation, at the end of the operation, it is imperative that the patient can have at least 10° hip extension compared to the fused spine. This is a prerequisite for post-operative spinal balance and the ability to walk without lumbo-pelvic limitation (Fig. 6).

  2. 2.

    The exploration of Extension Reserve will identify patients who have a hip extension limitation particularly as this does not appear on the full spine radiographs. In this case, an arthrodesis correcting defects of sagittal balance up to the neutral position can cause a failure because it will consolidate a limited extension reserve while removing the ability to compensate with the lumbar spine. Thus, it appears necessary to carry out a more important lordotic correction.

Fig. 6
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Kyphoting position on the left, lordoting position on the right

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Conclusions and Perspectives

The Extension Reserve appears as an important factor that must be integrated into diagnostic approaches, into surgical correction planning and also into physiotherapy programmes for low back pain patients.

This should also be considered by hip, pelvis and knee surgeons, especially whenever planning a surgical treatment.

In the prevention of low back pain, it is also possible to explore the Extension Reserve to track individuals who represent limitations and avoid back pain that develops later by implementing rehabilitation programmes either through self- or supervised rehabilitation.

The correction formula can help with more precision in the development of the operative strategy for sagittal imbalance.