Despite that normal values for the hip joint are reached at the end of ultrasound-monitored-treatment, the development of the acetabulum can be compromised during the growth phase. The acetabular index (AI) measured on a pelvic radiograph has been proven to be a reliable parameter. The aim of this study is to gain a better understanding of the dynamics of once-treated, residually dysplastic hips. This should be achieved by radiographically following these hips up to a milestone-examination at the end of preschool age. A total of 120 hips of consecutive 60 infants were included in this examination, each presenting with a residual developmental dysplasia of the hips (DDH) after successful ultrasound-monitored harness treatment. Radiographic follow-up was assessed retrospectively around 18 months, 3 years and 6 years of age, and the AI was measured. The age-dependent Tönnis classification was applied. The hips were assigned normal, mildly or severely dysplastic. Dependent t-test for paired samples indicated a highly significant improvement of the AI-values, including from the first to the second and from the second to the third follow-up. The percentage distribution into the Tönnis classification changed remarkably: in the first follow-up, 36 of the 120 hips were evaluated ‘severely dysplastic’, in the third follow-up only 1. On the other hand, three hips underwent acetabuloplasty. Even after normal values have been achieved at the end of ultrasound-monitored treatment, there remains a risk of residual dysplasia of the hips. Particularly, when the first radiographic examination shows nonphysiological findings, further close-meshed follow-up is recommended. Level of evidence: retrospective study of therapeutic outcome, consecutive patients, level II.

尽管在超声监测治疗结束时髋关节达到正常值，但髋臼的发育在生长期可能会受到影响。在骨盆 X 光片上测量的髋臼指数 (AI) 已被证明是一个可靠的参数。本研究的目的是更好地了解曾经治疗过的残余发育不良髋关节的动力学。这应该通过在学龄前结束时对这些臀部进行射线照相来实现，直到进行里程碑检查。本次检查共纳入 60 名婴儿的 120 髋，在超声监测的安全带治疗成功后，每个婴儿均出现髋关节发育不良 (DDH)。在 18 个月、3 岁和 6 岁左右回顾性评估影像学随访，并测量 AI。应用了依赖于年龄的 Tönnis 分类。髋关节被分配为正常、轻度或重度发育不良。依赖配对样本的t检验表明 AI 值有非常显着的改善，包括从第一次到第二次以及从第二次到第三次随访。Tönnis 分类的百分比分布发生了显着变化：在第一次随访中，120 个髋关节中有 36 个被评估为“严重发育不良”，在第三次随访中只有 1 个。另一方面，三个髋关节接受了髋臼成形术。即使在超声监测治疗结束时达到正常值后，仍然存在髋关节发育不良的风险。特别是，当第一次 X 线检查显示非生理性发现时，建议进一步密切追踪。证据级别：治疗结果的回顾性研究，连续患者，II级。