szmtag In Reply (23.10.2020)
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Dr Thaller correctly points out a conditionally incomplete presentation of risk factors for patellar instability and further names two important factors that substantially affect patellar tracking.

As mentioned in the introduction of the cited article, varus or valgus deformities of the leg axis in the frontal plane as well as tibial or femoral torsional deformities can contribute to the development of patellar instability and maltracking. Therefore, we mentioned these risk factors in our introduction and also visualized valgus deformity in Figure 1 (1).

Data on the importance of torsional deformities for patellofemoral instability are still inconsistent (2). We know from biomechanical studies that femoral deformities of 10° can already significantly destabilize the patellofemoral joint in case of a concomitant rupture of the medial patellofemoral ligament (3). No cut-off values have been established for valgus deformity so far. However, recently published case series support this association. Here, clinical results of the mentioned therapeutic options (varizating or derotational osteotomy) are clearly explained (4).

Aim of our study was to analyze the risk of recurrent patellar dislocation on the basis of predictive risk stratification models. Due to the study design of a systematic literature search, the risk factors displayed in our manuscript correspond to those of the included studies. Eight risk factors were identified, which were analyzed in predictive models. To improve clarity, age and skeletal maturity were combined under the risk factor “age.”

To our knowledge, no predictive models exists in the current literature that consider torsional or axis deformities. Including additional imaging modalities in our inclusion criteria would therefore not have affected study selection or the described risk factors. This observation rather indicates limitations of the analyzed studies, which was therefore discussed as limitation of our study.

For this reason, we agree with the valuable comment of Dr. Thaller and highlight the important impact of axis and torsional deformities on patellofemoral stability.

DOI: 10.3238/arztebl.2020.0733b

On behalf of the authors

Dr. med. Paola Kappel (geb. Koenen)

Klinik für Orthopädie, Unfallchirurgie und Sporttraumatologie

Krankenhaus Köln-Merheim

Universität Witten/Herdecke, Köln

paola.kappel@googlemail.com

Conflict of interest statement

The authors of both contributions of correspondence declare that no conflict of interest exists.

1.
Frings J, Balcarek P, Tscholl P, Liebensteiner M, Dirisamer F, Koenen P: Conservative versus surgical treatment for primary patellar dislocation—a systematic review to guide risk stratification. Dtsch Arztebl Int 2020; 117: 279–86 VOLLTEXT
2.
Balcarek P, Radebold T, Schulz X, Vogel D: Geometry of torsional malalignment syndrome: trochlear dysplasia but not torsion predicts lateral patellar instability. Orthop J Sports Med 2019; 7: 2325967119829790 CrossRef MEDLINE PubMed Central
3.
Kaiser P, Schmoelz W, Schoettle P, Zwierzina M, Heinrichs C, Attal R: Increased internal femoral torsion can be regarded as a risk factor for patellar instability — a biomechanical study. Clin Biomech 2017; 47: 103–9 CrossRef MEDLINE
4.
Frings J, Krause M, Akoto R, Frosch K-H: Clinical results after combined distal femoral osteotomy in patients with patellar maltracking and recurrent dislocations. J Knee Surg 2019; 32: 924–33 CrossRef MEDLINE
1.Frings J, Balcarek P, Tscholl P, Liebensteiner M, Dirisamer F, Koenen P: Conservative versus surgical treatment for primary patellar dislocation—a systematic review to guide risk stratification. Dtsch Arztebl Int 2020; 117: 279–86 VOLLTEXT
2.Balcarek P, Radebold T, Schulz X, Vogel D: Geometry of torsional malalignment syndrome: trochlear dysplasia but not torsion predicts lateral patellar instability. Orthop J Sports Med 2019; 7: 2325967119829790 CrossRef MEDLINE PubMed Central
3.Kaiser P, Schmoelz W, Schoettle P, Zwierzina M, Heinrichs C, Attal R: Increased internal femoral torsion can be regarded as a risk factor for patellar instability — a biomechanical study. Clin Biomech 2017; 47: 103–9 CrossRef MEDLINE
4.Frings J, Krause M, Akoto R, Frosch K-H: Clinical results after combined distal femoral osteotomy in patients with patellar maltracking and recurrent dislocations. J Knee Surg 2019; 32: 924–33 CrossRef MEDLINE

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