Neal JJ, Bowden DE The diagnostic value of panoramic radiographs in children aged nine to ten years. Br J Orthod. 1988; 15:193-197 https://doi.org/10.1179/bjo.153.193
Van der Weijden F, Dell'Acqua F, Slot DE Alveolar bone dimensional changes of post-extraction sockets in humans: a systematic review. J Clin Periodontol. 2009; 36:1048-1058 https://doi.org/10.1111Zj.1600-051x.2009.01482.x
Mamopoulou A, Hägg U, Schröder U Agenesis of mandibular second premolars. Spontaneous space closure after extraction therapy: a 4-year follow-up. Eur J Orthod. 1996; 18:589-600 https://doi.org/10.1093/ejo/18.6.589
Zimmer B, Guitard Y Orthodontic space closure without contralateral extraction through mesial movement of lower molars in patients with aplastic lower second premolars. J Orofac Orthop/Fortschr Kieferorthop. 2001; 62:350-366 https://doi.org/10.1007/pl00001941
Chalakkal P, de Ataide IN, Akkara F Modified serial extraction in a case with missing mandibular second premolars and a brief review of related treatment modalities. J Indian Soc Pedod Prev Dent. 2013; 31:126-131 https://doi.org/10.4103/0970-4388.115719
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Northway WM The nuts and bolts of hemisection treatment: managing congenitally missing mandibular second premolars. Am J Orthod Dentofacial Orthop. 2005; 127:606-610 https://doi.org/10.1016/j.ajodo.2004.12.001
Northway WM Hemisection: one large step toward management of congenitally missing lower second premolars. Angle Orthod. 2004; 74:792-799
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Agenesis of lower second premolars in an uncrowded dentition can represent a significant clinical challenge. Leaving the second primary molars and ultimate prosthetic replacement places a dental and financial burden on the patient. Therefore, space closure while often the preferred option, can be very difficult while controlling the overbite and position of the lower labial segment. Also loss of alveolar bone may make this impossible. Hemisection of the lower second primary molar can facilitate space closure by maintaining bone and if done at the correct time, allow the first molar to move mesially resulting in spontaneous space closure. This article describes this technique and illustrates it clinically.
CPD/Clinical relevance Hypodontia of the lower second premolars is a common problem and space closure to negate the need for expensive prosthodontics can be challenging. Hemisection of the second primary molar is a relatively simple procedure that can make space closure more predictable.
Article
Lower second premolars have been reported to be the most common missing teeth after third molars in non-syndromic hypodontia.1 In an uncrowded dentition, their absence represents a specific clinical challenge: the decision of whether to try and maintain the second primary molar, or whether to extract it and either prosthetically replace the missing second premolar, or attempt space closure. While each of these options has pros and cons, which are beyond the scope of this article, if successful, space closure will significantly reduce the dental burden created by placement of a resin-retained bridge or implant on the patient. However, controlled closure of the 9–10 mm of space created by extraction of a lower second primary molar, in an otherwise uncrowded dentition typically found in these patients, can be clinically challenging. Therefore, the purpose of this article is to demonstrate how space closure can be helped with the use of the simple and elegant technique, hemisection.
One of the main problems following extraction of a lower second primary molar is bone loss impairing space closure. Van der Weijden et al showed that extractions cause a mean reduction of the alveolar process, owing to bone resorption of 3.9 mm in width and 1.7 mm in height.2 Moreover, after extraction of primary second molars, there is often tipping of the neighbouring dentition into the extraction site, as well as mesio-lingual rotation of the first permanent molar.3 Another problem after extraction is spacing of the anterior dentition caused by distal migration towards the extraction site. This can also affect the lower midline and the overall symmetry of the lower arch. Taking the above into account, it would appear logical to postpone the extraction of the primary molar for as long as possible (after leveling and aligning is complete), in order to mesialize the permanent molar as much as possible, before the alveolus of the primary molar has become substantially resorbed.4
Bilateral space closure in cases with a moderate or severe anchorage requirement (such as severe anterior crowding and/ or proclination of the lower incisors) can usually be routinely and successfully treated with extraction of the second primary molars.5 However, in uncrowded or only mildly crowded cases, control of the position of the lower incisors and overbite can be very challenging; often requiring use of anchorage support anteriorly in the form of skeletal anchorage or fixed Class II correctors.6,7
Hemisection of the primary molar may reduce the need for these techniques, or even make them obsolete. Hemisection is not a new technique, having been originally described by Frans van der Linden in 1990,8 and has more recently been described by Northway and Valencia et al.9,10,11 Despite this, the technique is still rarely used.
Methods
Hemisection in the context of space closure means sectioning the primary molar and removing the distal half of the crown and the distal root. The primary molar is cut with a flame-shaped diamond bur in a high-speed hand piece, followed by extraction of the distal segment. Subsequently a pulpotomy/pulpectomy is performed, and the pulp of the remaining segment is covered with Ca(OH)2 (Dycal). A root canal treatment is usually not necessary.9 According to Valencia et al, even a pulpotomy is not mandatory.11 In a co-operative patient, the whole procedure can be carried out under local anaesthetic.
The space created distal to the hemisected primary molar allows the first permanent molar to drift mesially (Figure 1). As the mesial part of the deciduous tooth remains is left in situ, this prevents resorption of the alveolar bone and the width of the alveolar ridge is preserved.10 The typical hourglass-shaped resorption of the alveolus after early extraction of a primary molar is not seen (Figure 1). As the width of the alveolus is maintained, the first permanent molar tends to move forward bodily, as opposed to tipping and rotating. Timing is important, as for maximum space closure the hemisection should ideally be carried out prior to the eruption of the lower second molar, because its eruption will provide a mesializing force to the first molar.
Figure 1. The space created by the hemisection allows the first permanent molar to drift mesially.
At the same time, the remaining mesial part of the primary molar acts like a bookend, preventing distal migration of the anterior dentition and a midline shift. As soon as the first permanent molar is in contact with the remaining portion of the primary molar, the mesial segment is also removed to allow further space closure. However, by removing this segment, the ‘bookend function’ will be lost and a small shift of the lower centre line may occur. Therefore ideally, the extraction of the mesial segment should be carried out after the lower first permanent premolar has erupted so that its position can be retained by a simple removable space maintainer. The space maintainer can be used until the first molar is in contact with the first premolar, or the placement of a fixed appliance, to complete space closure. Occasionally, complete space closure will occur without the use of an appliance (apart from the space maintainer) (Figure 2).
Figure 2. (a–h) Complete space closure following hemisection of the lower second primary molars without the need for orthodontic treatment.
However, as a rule, there is still some residual space after completion of the eruption of the second molars, which makes orthodontic treatment with fixed appliances necessary. However, the treatment will be significantly simplified and the time shortened compared to attempting space closure after the extraction of a lower second primary molar (Figures 3 and 4).
Figure 3. (a–e) Space closure following hemisection of both lower second primary molars in a case with agenesis of lower second premolars achieved in a three year period. Republished with permission from Kieferorthopädie. Germany: Quintessence Publishing, 2015.Figure 4. (a–e) After complete eruption of the 2nd molars in the mandible, the patient was treated with a multibracket appliance for 1 year and 8 months. The duration of treatment with the multibracket appliance was thus significantly reduced.
The intra-oral findings shown in Figure 4 after the end of treatment show a Class I occlusion of the canines with a correct midline and harmonious arches. Pre- and post-treatment radiographs are shown in Figure 5. The bodily movement of the lower first molars achieved can be seen on the panoramic radiographs. An added benefit was the space created for the lower third molars by mesialization of the first and second molars. The cephalometric radiographs before and after treatment show an unchanged and ideal torque of the lower incisors, as well as a good soft tissue profile (Figure 5).
Figure 5. (a–d) Pre- and post-treatment radiographs of the case shown in Figures 3 and 4. Republished with kind permission from Glockengießer J. Lückenschluss im Unterkiefer durch Hemisektion der 2. Milchmolaren. Inf Orthod Kieferorthop 2019; 51: 271–275. 10.1055/a-0995-6396
Discussion
Hemisection of lower second primary molars when there are missing second premolars can be very helpful when space closure is planned. The earlier hemisection is started, the greater the chances for a complete mesial drift of the lower first molar. Before deciding to close the space by hemisection, two conditions are important:
Treatment should be started as early as possible. Northway describes a mean age of between 9.6 and 10.5 years, Valencia et al recommend 8–9 years.10,11
The lower third molars on the side of aplasia of the second premolar should be present. Otherwise, if the upper arch is treated on a non-extraction basis, the upper second molar loses its antagonist and is likely to over-erupt. This should especially be a consideration if the second primary molar is free from caries or unrestored and does not show root resorption, ankylosis or infra-occlusion. Bjerklin et al showed that the probable survival rate of these teeth in adulthood is more than 90%.13,14
In cases where the wisdom tooth is missing and long-term survival of the second primary molar seems unlikely, space closure may still be beneficial to the patient. In these cases, there are three possible strategies to prevent the upper second molar over-erupting:
Incomplete space closure, leaving some residual space so that the upper second molar is opposed;
Distally extend the occlusal surface of the lower second molar with composite in order to create an occlusal contact for the upper second molar;
Extraction of premolars in the upper arch if space is required to relieve crowding or reduce an increased overjet.
Another aspect to be aware of is the possibility of late development of lower second premolars. Bicakci et al describe an example of a late developing lower second premolar in a girl at the age of 9 years.15 Aichinger even reported on a patient in whom a tooth bud of a 35 formed at the age of 15 years.16 Although these are extremely rare individual cases, an X-ray check and appropriate information from parents and the patient should nevertheless be provided.
Conclusions
If the patient is correctly selected and the procedure is indicated, the closure of the space in the lower arch in cases with agenesis of lower second premolars can be facilitated by hemisection of the lower second primary molars. This is a simple, minimally invasive procedure that allows for excellent and predictable results, which can significantly reduce orthodontic treatment time.
