Sagittal Balance

Fig. 4.1

Pelvic sagittal parameters

Contrary to pelvic incidence, pelvic tilt and sacral slope represent a positional parameter that varies with position and spinal deformity. Pelvic tilt is the angle between a line from the midpoint of the femoral heads to the midpoint of the sacral endplate and a vertical reference line. Sacral slope represents the angle between the sacral endplate and a horizontal reference line. These values are highly interrelated as pelvic incidence can be quantified as the summation of pelvic tilt and sacral slope (PI = PT + SS). Hence, alteration of PT or SS is associated with a proportional decrease in the other value to maintain a stable PI.

These values represent an important compensatory mechanism in spinal deformity in that patients with positive sagittal balance compensate increasing pelvic tilt in order to maintain upright posture. This mechanism relies on extension of the hip and intact and strong gluteal muscles and is an energy-inefficient process. As patients become older, their ability to compensate for spinal malalignment in the sagittal plane becomes compromised secondary to multiple factors including hip osteoarthritis, hip flexion contractures, and weak gluteal muscles (hip extensors). These factors all limit the pelvic compensatory mechanism.

As noted by Stagnara in 1982 and confirmed more recently, due to the wide variability of the so-called normal sagittal alignment, treatment is ideally tailored to the individual patient rather than creating a “one-size-fits-all” approach. The guiding principles are that the patient should be able to stand upright with minimum effort and the lumbar lordosis should closely match pelvic incidence within 10°. As such, proper attention to pelvic parameters is important to accurate understanding and planning of spinal deformity surgery and key to obtaining good results in these patients.

Sagittal pelvic alignment has also been shown to be important in spondylolisthesis. Patients with elevated pelvic incidence and its associated elevated lumbar lordosis place increased shear forces across the L5–S1 pars, thereby placing the patient at risk for development of spondylolysis. As such, a linear relationship has been shown between spondylolisthesis grade and PI. Sagittal pelvic alignment is important also in determining whether or not slip reduction is indicated. Despite the severity of slippage, normal posture is maintained if sacropelvic balance is maintained. When this becomes unbalanced with a retroverted pelvis, the patient develops forward sagittal balance and this may indicate a need for reduction [8].

4.2 Global Alignment

Numerous measurements exist to evaluate global sagittal plane alignment. The simplest and most commonly used reference point is the C7 sagittal vertical axis (C7SVA). This is drawn vertically down from the C7 vertebral body. The distance between this line and the posterosuperior corner of S1 is measured. Patients are considered to have positive sagittal balance when this line lies anterior to this point and negative when it lies posteriorly. This line normally lies 0.5 cm (±2.5 cm) from the posterosuperior corner of S1. The advantage of this measurement is that it is relatively simple and reliable and familiar to the majority of practitioners. An additional method of measuring overall balance is utilizing the T1 tilt angle. This is the angle between a line from the centroid of T1 to the center of the bicoxofemoral axis and the vertical plumb line. This measurement has the advantage of not requiring calibrated radiographs. T1 tilt angle has been shown to correlate with SVA with angles greater than 25° correlating with greater than 10 cm of positive sagittal imbalance and most importantly with patient-reported outcomes [9].

4.3 Imaging

Accurate measurement of these parameters depends on the ability to obtain high-quality radiographs. Full-length posteroanterior (PA) and lateral radiographs on 36-in. cassettes are crucial for complete radiographic evaluation of spinal alignment. Newer low-radiation biplanar radiography provides for excellent visualization with only 10 % of the radiation of conventional radiographs [10]. Visualization of the femoral heads is mandatory for calculation of pelvic parameters.

Patients should be instructed to stand with knees locked in extension to fully appreciate the sagittal plane while minimizing the effects of compensatory knee flexion. Additionally, position of the arms is key in obtaining quality radiographs. While arm flexion is required in order to properly visualize the spine, arm position also affects the apparent sagittal alignment of the spine. Such changes can be minimized using multiple techniques. One common method is to have the patient holding an IV pole or ski poles, which keeps the arms at a 45° angle with the weight of the arms supported by the poles [11]. This allows for passive elevation of the arms with minimal change in sagittal alignment. An alternative position is to have the patient hold the arms with their fists resting on the ipsilateral clavicles.

4.4 Outcomes

Sagittal plane imbalance is poorly tolerated and has dramatic implications for patients in terms of functional status and quality of life [12]. Patients with forward (indicated as +) sagittal balance must constantly exert energy to maintain a horizontal gaze and upright position. This results in markedly increased energy requirement during activities of daily living with increased oxygen consumption of up to 28 % and 60 % at 25° and 50°, respectively, of trunk flexion [13]. Part of this compensatory mechanism is knee flexion, which contributes to increased energy expenditure. Patients may complain of thigh and buttock fatigue as well as hip or knee pain in addition to their back complaints. In addition, pelvic retroversion and hyperextension of normal motion segments are utilized by the patient to maintain an erect posture and require muscular activity and energy expenditure.

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