20.1 Introduction

Recognizing sagittal imbalance in adult spinal deformity is of primary importance as restoring the patient’s horizontal gaze is important not only for less energy expenditure but also for normal social interaction. ¹ To achieve optimum outcomes when treating these pathologies, a good understanding of the principles of sagittal balance is needed as well as a good understanding of the compensatory mechanism employed by the patient to maintain a horizontal gaze. ² One cannot stress enough the importance of these mechanisms as failure to identify them could lead to mediocre surgical results. In fact, Kumar et al ³ found, in 2001, that even short posterior fusion has a great risk of adjacent segment disease (more than 50%) if the fusion is done in an unbalanced state, while the risk of this complication is lowered by more than 20-fold if the fusion is done in a perfectly balanced state.

Once a surgeon has accepted the importance of restoring good sagittal balance, they must answer several questions before embarking on the surgical journey. What is the compensatory mechanism used in this patient for restoring its sagittal balance? What was his or her normal profile before having the degenerative changes? What is the best surgical strategy to achieve an outcome with minimal complications? What techniques does the surgeon have in his armamentarium to achieve his surgical goals? What are the expected outcomes of this surgery?

Based on angles reciprocity, American authors demonstrated that a good correlation between the pelvic incidence (PI) and the lumbar lordosis (LL) T12-S1 may be a basic requirement to obtain a good clinical result by using control with health-related quality of life (HRQoL). ^{4 ,​ 5} For a period of time, this theory was becoming a “credo,” with alternating good and bad results, mainly proximal junctional kyphosis (PJK). Even if many attempts at an explanation have been made, it seems that the intrinsic spinal shape was not well understood until now. Based on the normal sagittal shapes and their possible pathological evolution, we would like to propose treatment options for restoring the pelvic shape evaluated by PI.

20.2 From Normal to Pathological: Classification of Pathological Shapes of the Aging Spine

As presented in Chapter 6 in this book, Roussouly et al evaluated the spinal shapes in an asymptomatic population. ⁶ The authors defined four types of normal spinal shapes and they recently updated their classification to include a fifth type ⁷ (Fig. 20‑1). Recognizing normal sagittal profiles is important as it may help the surgeon to identify its degenerative evolution. Thus, every pathological sagittal shape could and should have come from one of the five described shapes depending on the PI.

The first phenomenon of local degeneration is the disk height loss with subsequent imbalance as a result of local kyphosis. As the compensation end point is having an upward gait and horizontal tilt of the head for a normal sight, several mechanisms for spinal compensation are observed as shown by the study of Berthonnaud et al. ⁸ If the spine is flexible, there is an increased extension of the flexible spine above and/or below this local kyphosis with contraction of the posterior spinal muscles, which lifts the trunk vertically, requiring painful abnormal effort from the spinal muscles to prevent falling forward with increased pressure on the posterior facets (Fig. 20‑2). If the spine is rigid, with progressive kyphosis, the gravity line moves forward and the pelvis rotates backward (retroversion), inducing a decrease in the sacral slope (SS) and increase of pelvic tilt (PT). ² Hyperextension of the hips follows, which is limited by the extension reserve (generally 10°) and, finally, a flexion of the knees in severe cases, controlled by the quadriceps. ²

It is fundamental to recognize both mechanisms. They have different clinical expressions and may be divided into local compensation and global unbalance. The first one (extension above and below the flexion accident) induces local compensations in the shape of the spine to restore a new alignment allowing slight global adaptations. It could be, for instance, an L3-L4 hyperextension above an insufficient lordotic L4-S1 arthrodesis, or a flattening of thoracic spine above a hypolordotic lumbar fusion. However, these local compensations are painful, both by local hyperextensive stress on the posterior facets, creating sometimes, at its maximum, a retrolisthesis, and by muscle contracture to maintain an abnormal local positioning as thoracic hypokyphosis. These patients are difficult to diagnose because they may have pain and discomfort without global unbalance, but, nonetheless, they have balance troubles.

The second mechanism is the well-known classical unbalance: loss of lordosis and/or hyperkyphosis, where compensation mechanisms are unable to balance the system, overpassed or, because of rigidity, retroverted pelvis, and forward displacement of the C7 plumb line (C7PL). This mechanism of pelvic retroversion is linked to PI value by the relation: PI = PT + SS. The higher the PI, the higher the ability of PT to increase. This mechanism has a double effect: horizontal positioning of SS and backward positioning of the whole spine regarding the femoral heads (FHs). This phenomenon is limited by the hip reserve of extension. ⁹ Maintaining this position is not economical and may induce muscle contractures (gluteus and hamstrings). Even if the balance seems controlled in a standing position, when walking, the expression of forward unbalance is stronger as a result of hip positioning in gait during a posterior step. When the spine is rigid, the forward unbalance provides discomfort but little pain.

Based on this, and on the Barrey ratio (BR), in a first attempt of classification, Le Huec proposed to divide sagittal spinal balance into three categories (Fig. 20‑3) ¹⁰ :

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  Type A or normal spinal balance: characterized by global balance of the trunk measured BR <100% (C7PL behind or just in front of the sacral plateau) and normal 10°<PT<25°. Lower limbs are completely extended in the standing position.

  
  
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  Type B or compensated balance: global balance of the trunk is still normal, BR is less than 100% (C7PL behind FH), but the pelvis is retroverted (PT >25°). The lower limbs show extension of the hips (femur straight) and the knees are in full extension.

  
  
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  Type C or decompensated balance: global balance of the trunk shows a positive C7PL, with a value that falls generally in front of the FHs (BR >100%), and the pelvis shows a PT in retroversion. In the lower limbs, there is extension of the hips (pelvic retroversion) and flexion of the knees. Hip extension is overpassed.

  
  
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  A fourth type could be added by describing the association of a lumbar hyperlordosis and an anteverted pelvis (PT >5°). Hips are in normal position or in slight flexion and knees may be in recurvatum. There is frequently a compensation in thoracic hypokyphosis. This is a frequent finding in anteverted type 3 spines (Fig. 20‑4).

  

  

It is important to differentiate sagittal balance or compensated balance. It was found that posterior fusion in a compensated balanced position was associated with increased adjacent segment disease compared to fusion patients in a balanced position. ³

Even though this classification is simplistic, one could not plan a surgical strategy based on this classification and no attempt was made to classify sagittal imbalance beyond this primitive reasoning. In fact, many attempts have been made to classify degenerative spine diseases, like the degenerative spondylolisthesis classification ^{11 ,​ 12} or adult deformity (Schwab-Scoliosis Research Society [SRS] classification). ¹³ The Schwab-SRS classification defines four types of basic deformities: T for thoracic scoliosis, D for double scoliosis, L for lumbar scoliosis, and S for sagittal deformity without coronal deformity. Added to the basic deformity are three modifiers: PT, PI-LL, and sagittal vertical axis (SVA) (Fig. 20‑5). All the parameters used in these classifications are positional parameters without any attempt to analyze the shape of the spine and its pathological evolution. In addtion, HRQoL scores and mechanical complications (PJK, nonunion, etc.) were correlated to some local parameters (PI-LL) or overall parameters (T1 pelvic angle, global tilt) without taking into consideration the shape of the degenerative spine.

Recently, we have proposed a classification and degenerative evolution of the aging spine based on the initial Roussouly classification and the compensatory mechanisms to local kyphosis (Fig. 20‑6, Fig. 20‑7, Fig. 20‑8, Fig. 20‑9). This study was done by analyzing a cohort of 331 nonoperated degenerative spines to identify possible degenerative evolution of normal shapes. ¹⁴ To make this classification easier to understand, it is divided into 11 types clustered into four categories (Table 20‑1):

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  Classical types: the described types 1 to 4 of the initial Roussouly classification (see Chapter 6).

  
  
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  Anteverted types 3 and 4: these types have, respectively, the same characteristics of the classical types 3 and 4 (medium and high SS) but with a low PT (PT ≤ 5°) (see Chapter 6).

  
  
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  Retroverted (or false) types: these types arise from the four classical types with a pelvic retroversion. False types 2 and 3 have, respectively, the same characteristics of the classical types according to the SS and LL shape, but they have a high PT (PT ≥ 25). False type 2 with thoracic kyphosis (TK) has a low SS, straight LL, high PI, and high TK.

  
  
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  Kyphotic types: when all compensatory mechanisms are consumed, kyphosis occurs. It can be divided into two subtypes according to the possibility of compensation in the thoracic spine:

  
  
  
  
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    Lumbar kyphosis shape: characterized by lumbar kyphosis with a hypokyphotic thoracic compensation curve (BR <100%).

    
    
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    Global kyphosis shape: characterized by lumbar kyphosis without a hypokyphotic thoracic compensation curve (and thus an unbalanced spine, BR ≥ 100%).

    

    

    

    

To simply understand this classification of spinal degenerative evolution, one must bear in mind the appropriate compensatory mechanism for local kyphosis: hyperlordosis of the adjacent mobile spine and/or retroversion of the pelvis. Considering these facts, this description of the sagittal alignment of the degenerative human spine is created by adding kyphosis to the four classical types, depending on PI.

Related posts:

22 Adult Scoliosis Treatment with an Anterior Approach 21 Sagittal Balance Treatment Strategy in a Posterior Approach 23 Techniques for Spine Osteotomies and Clinical Applications 24 Surgical Failures Mechanisms and Their Treatment 17 Advantages and Limitations of the SRS-Schwab Classification for Adult Spinal Deformity 15 Scheuermann’s Kyphosis

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