The incidence of adult spinal deformity is increasing as the population ages. Spinal deformity patients are typically evaluated with long cassette 36 in. x-rays to assess a variety of radiographic parameters. Relative radiographic indications to consider surgical intervention include a sagittal vertebral axis (SVA) greater than 5 cm, a coronal Cobb measurement greater than 20°, lumbar lordosis-pelvic incidence (LL-PI) mismatch greater than 10°, pelvic tilt (PT) greater than 25°, or a lateral listhesis affecting coronal balance to name a few [2, 9, 10]. These radiographic parameters must be accompanied by correlative clinical symptomatology of back and/or leg pain before surgery is recommended. The main goals for treatment of adult spinal deformity include decompression of the neural elements, establishing or maintaining sagittal and coronal balance, and achieving arthrodesis.

Traditionally, the adult deformity population has been treated with open surgical techniques. The morbidity associated with open techniques includes excessive blood loss and deep wound infection. Minimally invasive surgery (MIS) techniques have gained popularity with the goal of decreasing morbidity associated with open surgical techniques. However, initial MIS deformity correction efforts have often demonstrated under-corrected sagittal balance and pseudarthrosis at levels fused posterolaterally but not treated with an interbody fusion [11, 12]. Appropriate patient selection may have also contributed to early failures.

Early classification and treatment recommendation systems include the Silva and Lenke guide for adult degenerative spinal deformity scheme [10]. This scheme used six treatment levels to describe open techniques for the correction of adult degenerative scoliosis. Subsequent advances in minimally invasive techniques and adoption of their practice led to the creation of a modified scheme by Mummaneni et al. that included a combination of open and MIS techniques. This scheme was refined to a second algorithm presented here [5].

The minimally invasive spinal deformity surgery (MISDEF) algorithm was created to provide the framework for rational decision making when considering open versus minimally invasive techniques (Table 7.2). The MISDEF algorithm considers the strengths and weaknesses of a variety of minimally invasive techniques when compared to open techniques. Additionally, it attempted to address the complexity and poor interobserver reliability of previous algorithms. The MISDEF algorithm organizes three groups to simplify the algorithm (Figs. 7.1, 7.2, and 7.3). Progressively worse deformity leads to more aggressive correction approaches. Class I deformities in the MISDEF algorithm may be approached with MIS or mini-open muscle-sparing decompression alone or MIS fusion of a single listhetic level. Instrumentation, if placed, may be placed through an expandable port tube or through a percutaneous method. A Class II approach utilizes MIS or mini-open decompression and interbody fusion of the curve apex or the entire coronal Cobb angle of the major curve. This class of patients may benefit from a lateral interbody fusion in combination with a posterior MIS fixation strategy. A Class III approach entails a traditional open surgical approach involving osteotomies and/or extensive fusion including extension up to the thoracic spine.