Association between laterality and cleft width in patients with unilateral cleft lip and palate - a systematic review

In the complex landscape of human development, the face is a masterpiece of precise timing and coordination. Sometimes, the tissues that form the upper lip and the roof of the mouth do not fuse completely before birth, leaving a gap known as a cleft. This condition, which can affect the lip, the palate, or both, is not random. Scientists have long observed that when a cleft appears on only one side of the face, it is far more likely to be on the left side than the right. This consistent pattern suggests that the body's blueprint for building a face has a built-in direction, influenced by a mix of genetic instructions and environmental factors. Alongside this directional pattern, doctors also pay close attention to the size of the gap. A wider gap in the palate is generally considered more severe, often making the necessary surgical repairs more difficult and increasing the likelihood that a patient will need additional operations later in life. While researchers have studied the side of the cleft and the width of the cleft separately for decades, a specific question has remained unanswered: does the side on which the cleft appears actually influence how wide the gap is?

A team of researchers from the University of Bristol set out to find the answer to this question through a systematic review, a rigorous method of gathering and analyzing all existing scientific literature on a specific topic. They began by casting a wide net, searching through two major medical databases, MEDLINE and Embase, for any study that had ever compared the width of the palate gap between patients with left-sided clefts and those with right-sided clefts. Their search covered records published from the mid-20th century up to early 2026. After removing duplicate entries, they were left with 95 unique records to examine. The team then carefully screened the titles and abstracts of these papers, narrowing the list down to 21 full-text articles that seemed potentially relevant.

The researchers then read these 21 articles in detail, looking for a specific type of data: a direct comparison of palate width measurements between the two groups. They were looking for studies that had measured the gap in patients with left-sided clefts and compared those numbers directly to measurements from patients with right-sided clefts. Despite their thorough search and the high quality of the articles they reviewed, the team found that none of the studies contained this specific comparison. Every single one of the 21 papers was excluded because they either failed to report the width of the palate gap at all, or they reported the width without separating the data by which side the cleft was on. Some studies were also excluded because they were merely conference summaries rather than full research reports.

The result of this extensive search was a clear and somewhat surprising conclusion: there is no published evidence that directly investigates whether the side of a cleft lip is linked to the width of the cleft palate. The researchers did not find a single study that had successfully answered their question. This absence of data does not mean the relationship does not exist; rather, it indicates that the scientific community has not yet looked for it in a way that allows for a direct answer. The authors suggest that this gap in knowledge likely stems from how the data has traditionally been collected. While doctors frequently record whether a cleft is on the left or right, and they often measure the width of the gap, these two pieces of information are rarely analyzed together.

The difficulty in finding an answer is also compounded by the lack of a single, standard way to measure the width of a cleft. Different research teams use different landmarks on the anatomy to take their measurements, and they may use different tools or timing for when they measure. Without a universal agreement on how to measure the gap, it becomes very hard to compare results from one study with another, even if they were looking at the same thing. Because of this inconsistency, and because researchers have historically treated the side of the cleft as a simple description rather than a variable worth deep analysis, the potential link between the two has remained unexplored.

This systematic review highlights a significant gap in the current understanding of cleft lip and palate. The authors conclude that while both the side of the cleft and the width of the gap are well-known characteristics, their relationship has not been examined. They argue that for future research to uncover whether left-sided and right-sided clefts are truly different in terms of severity, the medical community needs to adopt more consistent methods for measuring and reporting these dimensions. By standardizing how these measurements are taken and ensuring that data is broken down by the side of the cleft, future studies could finally determine if the direction of the cleft holds clues to its size, potentially leading to more tailored and effective surgical plans for patients.